I think you're going to get some very frustrated players if you "cheat" to take away their toys...doubly so if you hijack another party member to do it. Finding clever solutions to problems within the ruleset is part of the fun, and from that perspective your Ranger is winning the game. You should be looking for ways to build on that while keeping the game fun, rather than trying to remove the complication.
Ideally, The poison should be saved for a "moment of glory", rather than providing a flat DPS increase. Keep it interesting.
One way to make that happen by reducing the number of poison vulnerable targets, while making sure there are a few that draw fire. For example, a necromancer with powerful undead guards - the Ranger is in a position to drop the caster, but needs the rest of the party for protection from the Skeletal Rhinos. High-Con enemies might be able to shake it off, or stay in the fight long enough to cause problems. Spirits, Elementals, Aberrations, anything that doesn't have a proper biology might change the game.
Another way to raise the value of the Poison is through attrition. Keep track of how many doses remain, and put the Ranger in a position to waste them on packs of bandits or raiders. Once they realize they could run dry, the problem solves itself - the Ranger will save the remaining Poison for boss fights.
Finally, use the Poison as an opportunity to advance your story and build your world. What if your characters enter a large city, only to find out that someone important was just assassinated with Wyvern Poison? The local authorities are desperate to find someone to pin it on, and if the PCs slip up & get caught with the poison things could get very dicey. The revelation that the VIP was killed with Wyvern Poison has made it a very hot commodity, and the local crimelords and nobles are all trying to score some for themselves, so you could make a tidy profit in coin or favors...or get double-crossed. Plus, behind all of this drama and scheming, somewhere there is the real assassin, and who knows what their next move might be.