Hi Dan - you are better asking this sort of question via support on the need help option. Support are then able to log in to advise you properly.
If these are the only entries in your books then you can "Reset" in settings which gets rid of all accounting entries, but you mustn't use this if you have other transactions you need from this or past years.
I'm afraid there's not a mass delete and so you may end up having to delete them one at a time. Having learned from bitter experience, you get into the habit up using backup before any major work like this so that you can restore should it all go wrong.
If you want to leave the paid ones on there then you can import a file of Sales Payments to off set them
All the best