What stops alpha decay?
Radioactive decay can't be stopped.
Decay of unstable atoms (radioactivity) is a spontaneous process that can't be stopped. That's also why half life can be determined, it is an intrinsic property. Even changing temperature, pressure, chemical composition etc. will not change the decay rate.
The only option is to shield the source of alpha radiation. This is actually quite easy, because the particles are large (in atomic terms). You can already stop them with a piece of paper. This doesn't mean they have less energy, it means they lose their energy within a very small range.