Please upgrade here. These earlier versions are no longer being updated and have security issues.
HackerOne users: Testing against this community violates our program's Terms of Service and will result in your bounty being denied.
Flagg inbox PM
There is no way to flag and report PMs (conversations) because they are confidential. Users should contact an admin about any users who send bad PMs.
Add Pages to Vanilla with the Basic Pages app
Users can include others in any conversation they are part of, so you could ask them to include a mod/admin if they are unhappy about a message.