Some tags seem to attract a significant amount of off-topic questions.
Specifically, we have tags for specific programming languages like c, c++, java, python, prolog which seem to be used frequently on off-topic questions. This happens even if the tag description explicitly mentions that programming questions are off-topic. Newcomers simply do not read the description.
In the past, it was suggested to simply remove such tags, but it was decided not to do so, since the lack of a tag would hardly stop the off-topic questions, who would use other tags. Indeed, some off-topic programming questions often misuse programming-languages instead, consequently notifying its followers / subscribers.
This problem might also affect other tags than those above, in other areas of CS.
(Please do not re-open the discussion for deleting such tags here. We are not discussing that.)
This problem could be mitigated if the UI was improved to somehow warn new users.
Here's a possible design. A tag can be marked as a "problematic" one by moderators. When a low reputation user is about to ask a question with a problematic tag, the UI shows a confirmation dialog titled "Your question might be off-topic!", highlighting the tag description (or wiki?), and asking the user to confirm the submission. Tag descriptions/wikis for those tags would be updated with this in mind.
A simpler alternative would be to warn all new users on all tags, or to make the tag descriptions more prominent somehow.