Well, I think frogs and lizards are probably your best defense against things like spiders, but I wouldn't be surprised if they were what was attracting the snakes too. Tough break - you're really between a rock and a hard place. A pet mongoose is out of the question, I take it?
I've built habitat gardens for people before to attract snakes; around here they're great for controlling rodents and such. (And for mosquitos, dragonfly gardens - much more beautiful than bug zappers.) Maybe you could do some research on local snake species (or whatever the skin came from, if you can ID it) and build an inviting trap so you could remove them? Or, if you can find a safe snake that occupies a similar niche to poisonous varieties, you might be able to squeeze the undesired one out by introducing a territory competitor.
I'm guessing you've come to terms with the black widows, but the same principle applies; if you can find a predator species, build a little habitat and invite them into your greenhouse as a control. There's always the vacuum-cleaner method, too, but you might want to rig some sort of special attachment so as to not rough up your plants too much.
~Joe