[Wilczek]

Here you go.

PHP Code:

#pragma semicolon 1
#include <sourcemod>
#define PLUGIN_VERSION    "1.0"

new Handle:g_hLocked = INVALID_HANDLE;

public OnPluginStart()
{
    AddCommandListener(Command_JoinTeam, "jointeam");
    g_hLocked = CreateConVar("sm_lock_teams", "1", "Enable or disable locking teams during match", FCVAR_NOTIFY);
}

public Action:Command_JoinTeam(client, const String:command[], args)
{    
    if (client != 0)
    {
        if(IsClientInGame(client) && !IsFakeClient(client))
        {
            if (GetClientTeam(client) > 1 && GetConVarBool(g_hLocked))
            {
                PrintToChat(client, "\x01 \x07You cannot change your team during a match!");
                return Plugin_Stop;
            }
        }
    }

    return Plugin_Continue;
}