I'm going to assume that this was in a ChitChat/Ping. There is/should be a button on that screen to report them.
You can also send a message to Waze Support and they can check the logs and send warnings if needed.
- Go to http://www.waze.com/support/ and search for "report a user".
- At the bottom of the results find "Still can't find what you're looking for? Click Here" & click there.
- Select "General"
- At the bottom of that results screen find "If you cannot find your answer, Please click here. " & click there.
- At the top where it says "General", replace with "Report a user" and fill out the rest of the form, being as detailed as possible. Give the username, date & time, and approximate location.
We know this is complicated, but it's configured this way to prevent people from just emailing common questions with out reading the documentation/FAQ where those questions are clearly answered.