I've submitted several proposals for flaws but I wasn't aware that the VeraCode web UI keeps them in "memory" until I commit them. So what happened is that I now have 100+ flaws which contain 7 proposals (the first one is the one for the flaw itself, the 6 others were created by selecting the next flaw and proposing for it).
How I can I mass "unpropose" or clean all my flaw proposals (i.e. start over)?
It's possible to remove proposed mitigations one at a time, but to remove a whole bunch of them at once I'd suggest you contact Veracode support.