Hi, unfortunately it is not possible to set the rule to the server, each player can deactivate/activate it with the command: /audiomsg .
You can always clean up the chat if you want, using something like this:
ClearPlayerChat(playerid)
{
for(new i=0; i < 11; i++) {
SendClientMessage(playerid, -1, " ");
}
return 1;
}