Assuming you moved the whole folder and not just the textures to the override folder, look in /override/NWNCQ_override_etcetc, and find a little batch file just called [cfgnwncq]. This is a little configuration utility that you can use to "turn off" individual tileset overrides. You should just be able to type a lowercase letter corresponding to the forest tileset, press enter, and it will move all associated textures to another folder until you want to turn it back on, in which case it will just move them back.
(Edit - barring that, you can manually remove the files associated with that tileset, but it's pretty quick to type a letter!)