1> Run a few adventures with creatures with a high level of poison resist (but don't tell the ranger player they have high resist, it could be an enchantment rather than a innate ability, particularly if they have heard of the party), till he runs out. Wyvern poison doesn't work too well against Wyverns for example :)
2> Have him make a roll when trying to poison arrow tips in combat. On a critical failure...... whoops! :) As poison doesn't last two long on arrow-tips, that should encourage him to keep a max of 1-2 arrows poisoned prior to each fight rather than poisoning every arrow.
3> Have some local NPCs die of poisoning, a local lord's son/wife are good candidates. Suddenly everyone with any type of poison that produces similar effects are under suspicion, if its a well loved ruler then it might be more so. After a few encounters where the ranger is having to plead his innocence, it might be more trouble than it's worth to keep the poison.
4> Move them to an a kingdom where poisons are banned. Maybe there was a spate of poisonings, perhaps it only applies to certain types of particularly powerful poisons that had been used in assassinations. Either way having to constantly hide it from the law, more so if they are having to deal with royalty may again make it more hassle than it is worth.
5> Put them in an enviroment where the poison isn't going to survive.
How long in the desert before it dries up completely? Put them in the
arctic... when he goes to poison his arrow, point out the poison has
frozen solid. Him trying to defrost it in a hurry could be interesting ;)
6> If he has THAT much poison, how is he storing is?
Could it spring a leak? (perhaps very weakly poisoning the lake they are next to, for comedy value giving everyone sudden case of the runs :) )
7> How much is that much Wyvern poison worth? Perhaps he could be persuaded to sell it or trade it for something all the party can use. Perhaps a scroll of group immunity to personification before they go fight as basilisk or similar.
8> Is there an antidote? Is it expensive? If not then again, if his enemies have heard of him and might suspect he is coming then they would probably invest in a few doses of this each! Or at least for their biggest guards/minions.
9> Effects from prolonged exposure. He might be using it 'safely' but hes firing the poison at high speed. The poison could be accumulating in his system so each time he uses it he has to make an increasing difficulty con check or start suffering side affects (shaking, sickness, fever).
Hope one of those help.