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Antivenom is the only thing I've found, and that's merely a +2 vs the effect of the poison.

It's not a disease to Cure Disease with, nor is it an effect listed under Remove Affliction.

Have I missed any other cures for poison effects a PC just can't shake using saving throws? 

If you are creating a custom poison that does not allow saving throws, then you would have to create a custom cure....

I am aware of that much.

My question was about poisons that exist, and no matter how many times a PC rolls, he just can't manage to save. 

Is there any potion or ritual available that simply says, "Ends the effect of one poison on target?" 

Cure Disease / Remove Affliction. 
Drastically more expensive option is Keoghtom's Ointment. Level 12 Wondrous.

Don't think there's anything else. 

Cure Disease and Remove Affliction do not work.

But thank you for pointing out the Ointment. That will prove most useful. 

I'd be inclined to say that poison counts as an "enduring effect", even if it's not specifically called out.  Although if the poison is dealing ongoing damage, by the time you could cast Remove Affliction, it's probably a moot point.

Oh, not a poison that doesn't have a save-ends mechanic, just the player having abysmally poor luck.  Could just have as many people as possible give Heal checks ...

Not a consumable, but Periapt of Cascading Health helps even the most unlucky player remove effects.

There are some other items that give a bonus against poison effects if you wear them.

Viper Belt (level 5 item) gives you Resist 5 Poison, and once per encounter you can get +2 on a saving throw against ongoing poison damage.

Amulet of Physical Resolve (level varies) gives +2 to saving throws against effects with the Poison Keyword and also against effects which slow, weaken or immobilize.

But having other party members use First Aid may be the simplest.

  Also, I would say any Save Ends effect ends after 5 minutes, so unless its ongoing damage, just handle it that way.