Probably “clever use of game mechanics”. However I don’t think that is intended. Ignore is supposed to be IGNORE. The WoW friends list system is very very old though and a lot of things are sort of cobbled together. You would also have to prove intent to evade Ignore to target you personally for it to count as Ongoing Harassment.
Put them back on Ignore. Document it. If they go around it again to personally target you. Document that as well, put that new char/account on Ignore, and report as Ongoing Harassment. Having the dates/documentation of actions helps with the Ongoing Harassment case.
You can read more about it here.
If you would really like an answer from Blizzard, you would want to post on the Customer Support forum. That forum serves as an Information Desk with Support Forum Agents around during work times.
To move your own thread, select the edit icon (pencil) and from the drop down list that says “General Discussion”, select “Customer Support”. That should allow you to move your own thread to the CS forum where you are much more likely to get a Blue answer vs only player opinions.