Is there a way to stop/turn off the File Vault process on my iMac?

I know of no way short of letting it finish and then you can turn off. However, it should not be doing the process for months even with a slow drive so something is not right here. My 27" iMac took a few hours. Erasing and starting over would work but perhaps another user can provide another alternative.

Terminal command

It will require the user to blindly type in the Administers' Password for this computer

Blindly meaning it will not show what you have typed

sudo fdesetup status

The status will be either ON or OFF

Also, refer to another posing related to FileVault and specifically to posting by @ HWTech

Though, aspects are related to Laptops other informations would still apply to your computer if it is a Desktop computer

What year and model is your Mac?

An older Mac (2017 and older) will take a while to encrypt, but not as long as you are seeing. It must remain powered on. I think free disk space is important, too.

Newer Macs either have a T2 chip or Apple Silicon. Those should encrypt instantly because they are already encrypted. If you have a newer Mac, there is something wrong.