If you use this code you posted, it will still work for everyone else, but they won't be informed of their error 'till after their post hits the server.
Code:
if (strlen($message)>$postmaxchars and $postmaxchars!=0 and ($bbuserinfo[usergroupid]!=6) and ($bbuserinfo[usergroupid]!=5) and ($bbuserinfo[usergroupid]!=7)) {
eval("standarderror(\"".gettemplate("error_toolong")."\");");
}