Clear Mendeley online library
The easiest and most reliable way I've found is to go to the online library of the Mendeley web page, and select the items you want to delete while in All Documents, and choose delete from the toolbar.

This will move the items to the Trash, which you can then choose to manually empty as well.

The trick is that if you select Sync on one of your devices now, sometimes it gets confused and adds everything back to the online library rather than clearing everything off. So I suggest deleting everything from each device manually - uninstalling, deleting the Mendeley folder, and then reinstalling in the worst case.

I went through this once while testing adding entire conference proceedings (1000+ documents with PDFs) , the deciding that was a bad idea and trying to remove them. They proved to be a bit sticky and kept coming back, but after I went through the above they were finally gone for good.