I personally love these types of players. I find it enjoyable to let them use their creativity a little bit and then throw a complication at them. Sure, that was a poison fungus, but after production of the poison you find that it is contact poison. I much prefer the "say yes" + complication than a say "no". Match creativity with creativity. Saying yes does not mean that they have the last word.
I've actually done this before, and the result was far more fun that what the result of saying no would have been.
Case in point: In 2e AD&D, A war cleric venturing in a "Savage Prehistoric Lands" continent wanted to catch and tame a velociraptor. He loaded up with about three animal friendship spells and wanted to sneak off into the jungle to find and tame one. I told him "sure" and ran the situation. Alone (because the rest of the party thought it a stupid idea), he finds the Raptor, and begins casting. The Raptor, of course, finds him.
Round 1: Raptor wins init, hits him, blows his spell.
Round 2: He casts again, Raptor wins init again, blows his spell.
Round 3: Cleric wins init, casts the multi-round spell, Raptor hits him a third time, spoiling the spell, and the Cleric is VERY hurt by now.
ROund 4 & later: THe cleric busts out his Broadsword, fights furiously for several rounds, killing the raptor, hears the rest of the pack in the nearby woods, runs for his life.
He limps back into the fortified town, half-healed up, embarrassed and not saying a word to the rest of the party.
For years after that, the group still talked about that incident. Had he planned it a little better, maybe convinced a couple of the group to go with him, he'd have had a raptor pet; but he tried something, I played it out with its complications, and we had a memorable story!
(Admittedly, I fudged him getting away from the rest of the Raptor hunting pack a bit; had the group been with him, I would have added that complication.)