One way to balance regeneration might be to apply a constant ki drain when its in use, similar to when turbo is activated. Additional balancing might be to remove a character's ability to use any ki attacks, halve their effectiveness, or increase their ki/stamina drain.
Or you could make regeneration a constant drain on stamina and ki. Just a couple solutions; I actually like the idea of buu, or picollo regenerating.
Discs rarely hit, making a character immune to them isn't a good idea. I think Buu's regeneration (1.2) should be removed anyway.
Piccolo can regenerate any lost bodyparts as long as his head's intact, and Cell can regenerate any lost bodyparts from a single cell. Maybe this idea would apply better to one of them, but even then, discs are so easily dodged that it would render them even more useless.