I am quite happy with the Duplicacy web version, but how can I remove older backups when my backup drive becomes full?
Old revisions (versions) within a specific Snapshot ID, or entire snapshot IDs?
If the former – configure prune job. If the latter – it’s a bit more involved, but I suspect you did mean the former, let me know if not 
Yeah I was thinking about versions. I can’t find a way to do it in the GUI version. I have the CLI, but I don’t know if I should combine the two or how to go about this.
Sorry but where do I click to find “Add a new job”? I only have Storage, backup, schedule and restore.
Edit: Nevermind I found it under schedules.
So it’s not possible to do this manually ad-hoc?
On the schedules page:
Do you already have a schedule configured that runs your backup? You can add a job to that schedule — click the + in the top right corner of that schedule.
Or you can create a new schedule with different cadence (click + in the bottom of the page) and add a prune job there
Is it possible to run a prune manually once in a while? If I uncheck all boxes (mon, tue, wed…) does that mean it will never run unless I manually run it?
Yes.
It’s honestly easier if you just delete the snapshot/id directory on the storage directly, then tidy up with prune -exhaustive.
Otherwise, you have to target every revision number (-prune -id <id> -keep 0:1 would get rid of the bulk, but then you’d have to do a final prune -id <id> -r <revision> -exclusive, where revision is the last revision standing. And be careful not to run -exclusive when anything else is running.
Since you need to delve into the storage to know the revision, might as well do the easy method.