How to stop decryption of external drive
The first thing I would do is open Spotlight system preferences and disable indexing of the external drive. It is a privacy setting.

Next, disconnect from the Internet and restart the Mac with the external drive connected. You might need to free up 5 or 10 GB of space to let things have enough room to sort out the decryption. 

There isn't an abort that I'm aware of, but you can use diskutil cs list to get a reading on the encryption / decryption progress and plan if you want to let it run or if you'll need to take the external drive to another Mac to let it finish. 

If you have a second USB drive, you could install a clean OS onto that drive and hold option. Boot to the empty/vanilla OS on the USB drive and let the external decryption complete.