How do I stop FileVault's decryption?
You have to let it finish FileVault decryption entirely before you can turn it back on. This can take hours or sometimes days, so you'll have to be patient. Encryption also takes nearly as long. There's nothing to do but wait for it to complete.

If you have a MacBook, be sure to keep it plugged in, as FileVault wont decrypt or encrypt while on battery power.

You should still be able to use your computer while it decrypts/encrypts.