Is there an equivalent of heaven and hell in Buddhism? According to most sources, a soul stays in samsara until it concentrates enough karma to get yet another rebirth.

However, I failed to find any sources explaining if this process is eternal. In other words, what happens if a soul has committed so many sins that it's unable to accumulate enough karma? Or, in other words, is there a "dead end" when no more rebirth is possible?
There is no sense of accumulating sins, per se; your karma (actions) change who you are; there is also no sense that it can ever be too much to recover from (eventually). Eventually, your karma will catch up with you, sending you to heaven or hell, and making it more difficult to accumulate further good or bad karma, until you usually wind up right back where you started, somewhere in the middle. Something like the law of diminishing returns; the better or worse your rebirth, the harder it is to make further corresponding karma.

At any rate, given the infinitude of samsara, there is no such thing as too late; if there is time to accumulate good or evil, there is also time for its effects to wear off.

See the Assu Sutta, SN XV.03:


  From an inconstruable beginning comes transmigration. A beginning
  point is not evident, though beings hindered by ignorance and fettered
  by craving are transmigrating &amp; wandering on.