Well, this is a tough one. If it really is an impaction, it should be kind of mobile, you'd be able to move it around with your fingers. If it's not too big it may be possible to carefully palpitate it out after a soak. This requires you being very very gentle as you can hurt the snake doing this. But if the mass isn't huge it's not very hard. If you do this make sure you're not aiming the snake at anyone's face. At least not anyone you like. I've also had luck injecting small amounts (1mg/kg) of mineral oil into a dead prey item, or you can do this through stomach tube if the snake won't eat (and if you know how). Ideally you'd have a vet do this stuff, but it's possible to do yourself if the problem isn't too bad. Just don't go putting anything into the snake's cloaca. If none of this works there's no option but to find a herp vet somehow, this problem can occasionally require surgery.
(If you just got this animal, it's possible that the problem started before you had it, so don't beat yourself up too much)
((recommended reading, or 'how i learned this, at least': "What's Wrong with my Snake?". It has a pretty good section on this particular problem. You may have to buy it on Amazon.))
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Eryx - All the fun of a boa in a convenient pocket size!