This is a really annoying problem. The cancelling process is so convoluted it takes a full page to describe what does what (just look at this), and it can’t even abort things like a stuck upload.
Right now the best option to do what you want is to edit the backup(s), disable the schedule, then kill duplicati if stop now is stuck after a minute or so (hoping that doesn’t break something, if it does, a repair should fix it). Then, after a restart you can actually restore something without all the queued backups getting in the way. Afterwards you have to enable all the stuff again.