I agree with above suggestions. This might be a tough problem to correct but it's going to be worth it, and I'm really, really proud that you'd like to give it a go! :D
I'd ask local pet stores if they would take the guppies (and as Izzy said, try removing the girls first.) Aim at keeping only 5 or so males. If no stores are willing, try posting ads online that you have extra guppies to give away (specifically stating that they are not feeders, if that's not what you want.)
Once the excess guppies are gone, we can look at either doing some super hardcore siphoning of the substrate to get rid of the majority of the snails and whatever they're eating, or removing the substrate altogether if it hasn't been kept vacuumed (ie, lots of crud that's settled underneath it)
Are water changes done regularly on this tank?