Based on past experience, where a band member ended up in the ER due to a patron bringing her a spiked drink, we no longer touch anything that has not been delivered by bar staff.
It's our rule, and the rule at said club since then. Perhaps you could let the bar management/staff in on the situation. If the person is a regular, shouldn't be too much trouble to have them simply pour alcohol-free sodas when this dude steps up to the bar. I'd bet good money he makes them well aware who he's buying for.
Agree with most other methods listed so far to try and squelch this behavior.
Exception: Eye drops?
Being curious, here's a quick interwebs search/quote about it's effects:
"Swallowing even small amounts can lead to seizures, cardiovascular collapse, and coma."
Um, no, just don't do it. The guy's heart seems to be in the right place. Vengeful injury or possible death is just not called for.
(I'm sure it was meant as a funny, but I just had to know...)