You can’t do this directly in the GUI, you’ll need file access to the backup storage…
First thing I suggest you do is to make sure Duplicacy didn’t create any 0-byte files (either chunks or snapshot revisions) - because if a check sees them, it assumes they’re in good nick. Manually find and delete them first.
Run a check -persist then manually delete any snapshot revision files (under /snapshots in the backup storage) - these should hopefully be few and most recent only.
Ensure no backup jobs are running and run a prune -exclusive -exhaustive to free up space. You need a little space in order to run prune -keep in the next step, so if you already have a wee bit of space and it’s literally not 0 bytes free on the disk, you can probably skip this.
Run your desired prune -keep policy.
Finally, run prune -exclusive -exhaustive again to delete the marked chunks. (Just don’t ever run prune with -keep and -exclusive at the same time.)
Do another check to make sure everything is alright.