It is not possible to stop the decryption of the drive while also using the drive.
You can unmount and possibly unplug the external drive, and the decryption will obviously stop.
However, as soon as you plug the drive in again and mount the file system, the decryption process will continue. You cannot ask the system to stop that.
Your options are either to a) let the decryption process finish, and then apply encryption again -- or b) backup the files off the drive, wipe the drive, create a new encrypted file system and copy back your files.
Option (b) usually takes just as long as option (a), but requires a lot more manuals steps. So I would recommend option (a).