Terrible mole/vole problem in East Texas. The only way I’ve found to get rid of them successfully is to kill their food source (grubs). My husband likes the “golpher hawk,” it works really well, but you have to physically remove the mole when caught.
I put hardware cloth down over the entire floor area of mine, then covered that with gravel. (I installed it before putting the greenhouse on top, but you might be able to do it now, if you run it up the inside walls a foot or so.)