[eyal282]

#include <sourcemod>

// RESERVE_MAX_PLAYERS should be equal to the maximum amount of players allowed in the server without reservation. If the player count is equal or greater than this number and a ADMFLAG_RESERVATION joins, kicking will occur.

#define RESERVE_MAX_PLAYERS 20

public OnClientPostAdminCheck(client)
{
	if(!CheckCommandAccess(client, "sm_checkcommandaccess_reservation", ADMFLAG_RESERVATION, true))
		return;
	
	new TClients[MaxClients+1], CTClients[MaxClients+1], TCount, CTCount, PlayerCount;
	new TCanBeKicked = false, CTCanBeKicked = false;
	
	for(new i=1;i <= MaxClients;i++)
	{
		if(!IsClientInGame(i))
			continue;
		
		switch(GetClientTeam(i))
		{
			case CS_TEAM_T:
			{
				TClients[TCount++] = i;
				
				if(!CheckCommandAccess(i, "sm_checkcommandaccess_reservation", ADMFLAG_RESERVATION))
					TCanBeKicked = true;
			}
			case CS_TEAM_CT:
			{
				CTClients[CTCount++] = i;
				
				if(!CheckCommandAccess(i, "sm_checkcommandaccess_reservation", ADMFLAG_RESERVATION))
					CTCanBeKicked = true;
			}
		}
	}
	
	if(!TCanBeKicked && !CTCanBeKicked) // In this case, the entire server has reservation, nobody to kick lol.
		return;
	
	PlayerCount = TCount + CTCount;
	
	if(PlayerCount < RESERVE_MAX_PLAYERS)
		return;
	
	new KickTarget;
	
	if(TCount > CTCount || !CTCanBeKicked)
	{
		KickTarget = GetRandomPlayer(CS_TEAM_T);
		
		KickClient(KickTarget, "You have been kicked due to reservation.");
		
		ChangeClientTeam(client, CS_TEAM_T);
		
		return;
	}
	
	if(TCount < CTCount || !TCanBeKicked)
	{
		KickTarget = GetRandomPlayer(CS_TEAM_CT);
		
		KickClient(KickTarget, "You have been kicked due to reservation.");
		
		ChangeClientTeam(client, CS_TEAM_CT);
		
		return;
	}
	
	new TeamToKick = GetRandomInt(CS_TEAM_CT, CS_TEAM_T); // If this line gives an error, replace the locations of CS_TEAM_CT and CS_TEAM_T
	KickTarget = GetRandomPlayer(TeamToKick);
	
	KickClient(KickTarget, "You have been kicked due to reservation.");
		
	ChangeClientTeam(client, TeamToKick);
	
}

stock GetRandomPlayer(Team)
{
	new clients[MaxClients+1], Count;
	
	for(new i=1;i <= MaxClients;i++)
	{
		if(!IsClientInGame(i))
			continue;
			
		else if(GetClientTeam(i) != Team)
			continue;
		
		else if(CheckCommandAccess(i, "sm_checkcommandaccess_reservation", ADMFLAG_RESERVATION))
			continue;
	
		clients[Count++] = i;
	}
	
	return clients[GetRandomInt(0, Count-1)];
}

Try this: not tested or even tried to compile.

#include <sourcemod>
#include <cstrike>

// RESERVE_MAX_PLAYERS should be equal to the maximum amount of players allowed in the server without reservation. If the player count is equal or greater than this number and a ADMFLAG_RESERVATION joins, kicking will occur.

#define RESERVE_MAX_PLAYERS 20

public OnPluginStart()
{
	HookEvent("jointeam_failed", Event_JoinTeamFailed);
}

public Action:Event_JoinTeamFailed(Handle:hEvent, const String:Name[], bool:dontBroadcast)
{
	new client = GetClientOfUserId(GetEventInt(hEvent, "userid"));
	
	if(!CheckCommandAccess(client, "sm_checkcommandaccess_reservation", ADMFLAG_RESERVATION, true))
		return;
	
	new TeamChosen = GetEventInt(hEvent, "reason");
	
	if(TeamChosen == CS_TEAM_SPECTATOR)
		TeamChosen = GetRandomInt(CS_TEAM_T, CS_TEAM_CT);
	
	new TClients[MaxClients+1], CTClients[MaxClients+1], TCount, CTCount, PlayerCount;
	new TCanBeKicked = false, CTCanBeKicked = false;
	
	for(new i=1;i <= MaxClients;i++)
	{
		if(!IsClientInGame(i))
			continue;
		
		switch(GetClientTeam(i))
		{
			case CS_TEAM_T:
			{
				TClients[TCount++] = i;
				
				if(!CheckCommandAccess(i, "sm_checkcommandaccess_reservation", ADMFLAG_RESERVATION))
					TCanBeKicked = true;
			}
			case CS_TEAM_CT:
			{
				CTClients[CTCount++] = i;
				
				if(!CheckCommandAccess(i, "sm_checkcommandaccess_reservation", ADMFLAG_RESERVATION))
					CTCanBeKicked = true;
			}
		}
	}
	
	PlayerCount = TCount + CTCount;
	
	if(PlayerCount < RESERVE_MAX_PLAYERS) // No need to kick.
		return;
	
	else if(!TCanBeKicked && !CTCanBeKicked) // In this case, the entire server has reservation, nobody to kick lol.
	{
		PrintToChat(client, "Tough luck - everybody in the server has reservation.");
		return;
	}	
		
	new KickTarget;
	
	if(TeamChosen == CS_TEAM_T)
	{
		if(!TCanBeKicked)
		{
			PrintToChat(client, "Nobody in the terrorist team can be kicked.");
			return;
		}
		KickTarget = GetRandomPlayer(CS_TEAM_T);
		
		if(IsPlayerAlive(KickTarget))
			ForcePlayerSuicide(KickTarget);
		
		CS_SwitchTeam(KickTarget, CS_TEAM_SPECTATOR); // Since he's in spectator, we need no delay with certainty. 
		
		CS_SwitchTeam(client, CS_TEAM_T);
		
		KickClient(KickTarget, "You have been kicked due to reservation.");
		
		return;
	}
	
	else if(TeamChosen == CS_TEAM_CT)
	{
		if(!CTCanBeKicked)
		{
			PrintToChat(client, "Nobody in the CT team can be kicked.");
			return;
		}
		KickTarget = GetRandomPlayer(CS_TEAM_CT);
		
		if(IsPlayerAlive(KickTarget))
			ForcePlayerSuicide(KickTarget);
		
		CS_SwitchTeam(KickTarget, CS_TEAM_SPECTATOR); // Since he's in spectator, we need no delay with certainty. 
		
		CS_SwitchTeam(client, CS_TEAM_CT);
		
		KickClient(KickTarget, "You have been kicked due to reservation.");
		
		return;
	}
}

stock GetRandomPlayer(Team)
{
	new clients[MaxClients+1], Count;
	
	for(new i=1;i <= MaxClients;i++)
	{
		if(!IsClientInGame(i))
			continue;
			
		else if(GetClientTeam(i) != Team)
			continue;
		
		else if(CheckCommandAccess(i, "sm_checkcommandaccess_reservation", ADMFLAG_RESERVATION))
			continue;
	
		clients[Count++] = i;
	}
	
	return clients[GetRandomInt(0, Count-1)];
}