Preventing the back button from exiting your site is one sure way of losing traffic!!!! Don't you get pissed off when other sites do it to you???? Do you return to such sites??? Why do you think anyone would touch your site with a disinfected barge-pole if you do things like that???
Rant over. But seriously: If your site has a high bounce rate, that should be telling you something. Most likely:
- Your site either has a misleading URL, keywords or description (so people are visiting it who are not interested in it), or...
- It is crap!!!!
Either way, fix the problem not the symptom!!!