Balsamiq Edit: This has been implemented in version 3.4.5
It would be nice to have a simple way of cleaning all alternatives in one go. The current process of having to go to every page and then right-clicking in the Alternate Version list for every single alternate is quite a lot of work …
Hey again Phillipp!
I got this added from the other thread, but I should mention that if other folks would find this useful (or have suggestions for implementation), comment or like this thread!
Thanks again for the suggestion, Phillipp! 
Alright then, +1 for this feature. It’s pretty annoying to have to delete each one individually, especially when it has to render up each mockup first (which takes a couple of seconds itself) before I can rightclick to delete. Multiply that by the fact that I tend to duplicate mockups as I go to create clicky walkthroughs and I can end up spending an awful lot of time clicking-waiting-rightclicking to clean up my file.
Hi @PhilippQF and @stewartmurrie,
We just released our latest official version (3.4.5) and you can now discard all alternates for a mockup with a dedicated menu command:
Thanks again for the request! 
The new version (3.4.5) is available here.
And the complete release notes are here if needed.