This simple TShock plugin prevents players from changing their team.
Download the latest version of NoTeams.dll from releases and copy it into your ServerPlugins folder.
tshock.noteams - allows toggling NoTeams on and off and reloading the config.
/noteams on - turns NoTeams on;
/noteams off - turns NoTeams off;
/noteams reload - reloads the config.
enabled - can be "on" or "off"; determines if NoTeams is on or off at the start (can still be toggled with commands even if set to "off");
message - what player sees when he tries to switch teams;
kickOnSwitchAttempt - determines if the player should be kicked when they try to switch teams.