Yes, it would be nice if there was a way to stop the iCloud transfer. In my case it took about 36 hours for the transfer to complete and I was traveling at the time using my personal mobile WiFi (limited GBytes). I definitely did not want to use up all my data on this iCloud file transfer. So, in my case, what I did was to briefly turn on my mobile WiFi to allow a quick internet connection for, say, a web page - then I'd quickly shut off the WiFi to prevent the iCloud transfer from sucking up my limited data. I used the WiFi icon located at the top of of my screen (very convenient). Once at the hotel - I connected to their WiFi and let iCloud finish.
Although it was handy to move 90GB of dead weight on my MacBook - next time I'll be grabbing an external (and much faster) storage device for jobs like that. Cloud based storage is like a new technology - in that we need to adjust our workflows to accommodate as well as find out how the technology can best be used to our benefit.
EDIT: I haven't tried this yet - but it looks like you can stop the transfer by unchecking iCloud Drive (System Preferences --> iCloud)