What must be done before rebirth in the hell realms becomes inevitable? I once read a commentary by a Rinpoche talking about how the karma of killing a single fly grew at an exponential rate. 

Since negative karma grows so quickly, what actions, and what amount of those actions, makes rebirth in a hell realm inevitable?
Five offences in Buddhism which are said to irrevocably propel the offender into hell are:


patricide,
matricide,
killing an Arhat,
injuring a Buddha,
creating schism in the Sangha


Smaller offenses like common murder, adultery etc. can still be "repaired" even if with great difficulties.