You mean you wanted to block the team change command ? Here ,
register_plugin("Block Team Change", "1.0", "...");
client_print(id, print_chat, "[AMXX] You are not permitted to switch teams.");
Found this code @ Block Team Change
If you want something that auto makes players join a specified team , Then :
Auto Join On Connect.
Team Join Management.
** Please search google for the plugin which you needed before posting **