You can already assign specific stylesheets per forum but a users's stylesheet preference will override what you have set but this is only for them. Are you trying to take away the users's ability to choose a stylesheet to use? If so then go to your admin area and edit your theme settings. You should see this line:
"The following stylesheets were found in your stylesheet directory. Place a checkmark by those you want to make available to your users."
Just remove the checks from them so your users can't select them.
Of course I'm not sure if this willl affect the ability to assign the different stylesheets to different boards but I wouldn't think it would...
