Never mind, I resolved it. Session was last weekend so any input is to late anyway.
I ruled it would continue to heal until they a) moved out of range or b) try to dispose of it in an manner that I could approve off.
In the end they beat it down into negative HP, have it rise once, then beat it into negative again.
Then the tried to burn it (not very smart with a fire loving creature
) force Holy Water down its throat, beat it to -100 HP, behead it and dismember it. I ruled that it stopped healing right around then.