The usergroup or forum ID settings wouldn't really work on this, I have come to see. As anyone can use this, those settings would only make for a lot of empty replacement variables throughout the site. You can't stop anyone from actually typing the code in a post. All you can do is stop certain forums or usergroups from seeing the parsed variable, hence the code will show. What I do is use it only for staff and use a word that only I and the staff know. It saves for a lot of headaches and empty code tags throughout the site. I have added the option to change the word form the Admin CP settings in case someone decides to get annoying with this. Although I haven't found the way to stop the code from showing in the notification e-mails yet. I am working on that in between my site crashes.
You will find that this is a lot of fun to play with in the beginning but can get overused and really annoying if you let the wrong people use it.