Can you clean off spider venom from a sandal?

I wouldn't be worried about it. If I thought there might be some black widow venom I guess that I would throw both sandals in the washing machine.

Actually, just a little soap and water should remove it. Unless you are in danger of cutting your foot right at the point where there is some dried venom, there should not be any problem with leaving the venom where it is. Spider venoms are chemically complex, so it seems likely that UV in the sunlight, oxygen from the air, and maybe some other factors would make the venom lose its original chemical nature pretty quickly. I've never heard any reports of people getting hurt by accidental contact with any spider venom that wasn't delivered by way of a spider's fangs.

If fresh black widow venom were allowed to drip into an open wound, then I think you would feel it right away and leave your wound under a cold water faucet until the venom got diluted and washed away.

I have no idea what people who collect spider venom to make antivenoms may do to clean up their lab equipment, tabletops, etc. I suspect that a little Comet Cleaner or something else with chlorine bleach in it, or maybe an alcohol disinfectant, would be all that they would ever feel that they needed to use. But they would probably be worried more about the health of their Spiders than that somebody might come in with a sliced-open hand and rub the wound around on the lab table where they extract black widow venom. I think I remember reading that freeze dried spider venoms can be purchased for research purposes, so I'm inclined to believe that you either have wet venom or dry powder. Nothing I can every remember reading has suggested that the venom is sticky.

I think the real problem would be how to preserve venom on a sandal so that it would not lose its potency. For that the safe thing might be to put the whole thing in an airtight container and then freeze it or something. That's just a round-about way of saying that I don't think you should have any problem getting rid of the venom at all. I'd be surprised if washing the sandal under a garden hose didn't get all of it, but since you're worried about it, why not try pouring some Clorox on it? Then you could run it through the washing machine.

The only time I ever saw wolf spider venom that had been injected into a cotton pad it was dark brown. If you can see where the venom is, then squirting it with water would probably remove the colored stuff. If that didn't work then alcohol followed by a rinse and then some liquid bleach would probably remove the color. Beyond that I don't know how you could possibly prove that there was no venom remaining.