[unnyquee]

In case you might refer to what Brreaker said..

PHP Code:

#include <amxmodx>
#include <amxmisc>
#include <cstrike>

#define ACCESS ADMIN_BAN // your access level
#define isPlayer(%1) (1 <= %1 <= get_maxplayers())

new bool:g_bClientBanned[32];

public plugin_init()
{
      register_plugin("Ban for Joining CT Team", "0.0.1", "benamo6");
      
      register_event("TextMsg", "eJoiningCT", "b", "2=#Cstrike_TitlesTXT_Game_join_ct");
      
      register_concmd("amx_ctban", "_ctBan", ACCESS, "<nickname> <status> - sets whether <nickname> can join the CT team or not");
}

public _ctBan(id, level, cid)
{
      if(!cmd_access(id, level, cid, 3))
      {
            return PLUGIN_HANDLED;
      }
      
      new szNickname[32], szStatus[2];
      new iStatus, iTarget;
      
      read_argv(1, szNickname, 31);
      read_argv(2, szStatus, 1);
      
      iStatus = str_to_num(szStatus);
      
      iTarget = cmd_target(id, szNickname, 8);
      
      if(!iTarget || !isPlayer(iTarget))
      {
            return PLUGIN_HANDLED;
      }
      
      if(iStatus == 1)
      {
            g_bClientBanned[iTarget] = true;
      }
      
      else 
      {
            g_bClientBanned[iTarget] = false;
      }
      
      return PLUGIN_HANDLED;
}

public eJoiningCT(id)
{
      if(g_bClientBanned[id] == true)
      {
            client_print(id, print_chat, "* You have been banned from joining the CT Team.");
            client_print(id, print_chat, "* You have been moved to SPECTATORS.");
      }
      
      _moveToSpec(id);
}

_moveToSpec(id)
{
      cs_set_user_team(id, CS_TEAM_SPECTATOR);
} 


..Hope that will do the trick!

Non-tested!