@qui_g0n You should be able to safely End Task (Win) or Force Quit (OS X) the DiffMerge in this situation.

DiffMerge 4.2 is out: export diffs as HTML or text; improved diff visibility; better Finder integration on OS X. sourcegear.com/diffmerge/web…

DiffMerge 4.1 is out, with improved exports, better Find and Copy workflows, and loads of smaller fixes. sourcegear.com/diffmerge/web…