Depends on the hybrid and how closely related the races are
a human male wouldn't be able to have an offspring with a slime, period. An elf and a human are pretty closely related, so their hybrid species could be fertile. Now let's say that a dwarf and an elf were to have a child. Now let's say they are closely related, but not that close, so fertile hybrids between them are rare.
Now back on topic, a fantasy village fights them very easy. Use dirt, shit, and other fertilizer. Throw it on top of them, or dig holes and have them fall in them, then fill it back up.
Their core crystal things that keeps their form will be burried and it provides an endless fertile soil for many generations.
If you say they just become ground water, then you just admit they're part of the water cycle and will evaporate into the air....which might be a bad thing. It could be raining down slime girls, which like I said, can be good or bad depending on your setting