This is a part of how DnD works: as the characters gain spells and magic items in their tool set, planned plot lines will change. New appropriate ones pop up, old ones become obsolete. This is referenced in the Basic Rules:
In the first tier (levels 1–4), characters are effectively apprentice adventurers. ... The threats they face are relatively minor, usually posing a danger to local farmsteads or villages.
In the second tier (levels 5–10), characters come into their own. ... These characters have become important, facing dangers that threaten cities and kingdoms.
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Essentially, as Remove Curse is a 3rd level spell, available for characters of 5th level and beyond, it renders plot lines hinging on individuals fighting curses obsolete, similarly to how 5th level combat power renders the start-of-campaign miniquests about rats in the cellar obsolete. It's no longer a challenge.
What to do about this?
The simple answer of course would be not to found adventures on an objective the players have the means to easily bypass, but given the large amount of spells and other means available to your players, I understand it's not really foolproof unless one is really devoted to preventing such bypasses. Knowing every option available should not be a prerequisite for being a GM.
An alternative is always having a backup plan, so that if your players have the means to bypass a planned adventure, you have another one in storage. If you're good with improvisation, you can get by with as little as the next adventure hook, eg. something Fiachra wants to do after the curse is undone.
Depending on what your players actually want, though, you could simply discuss with them if they're ok with not using adventure bypasses. Many players prefer well-prepared content over "watching paint dry" so they might approve of the "level 42 wizard" explanation or similar tricks. Ask your players what they think works the best for them.