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This is really a response to Martin's answer (which can't be fit in a comment) regarding using the existing tag system to implement this.

I believe this is a very, very bad idea.

First, placing all-caps text in the tag wikis doesn't actually stop the tag from being used. During The great Stack Overflow tag/question cleanup of 2012 (10K users on SO, and I'm the moderator who coordinated that), we placed a disclaimer in the tag wikis of all the tags that were to be cleaned up and not used anymore during the cleanup:

DO NOT USE - Removed as part of "The great Stack Overflow tag/question cleanup of 2012" - See link in Tag Wiki for more information.

And the following was placed in the top of every extended description in the wiki:

DO NOT USE - Removed as part of http://meta.stackexchange.com/questions/128315/the-great-stack-overflow-tag-question-cleanup-of-2012.

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The reason this was done was because we wanted to stem the use of these tags as they were being removed, not to serve as permanent guideposts for what not to do.

During that time while somewhat effective, the notice at the top didn't keep the tag from being used; we still had questions being tagged with those tags (many have been cleaned up since then).

Even now, on Stack Overflow, with the homework tag officially depreciated, up until recently we were getting a few questions a day, despite it being tagged with featured on meta as well as placed in the community bulletin.

That said, I suggest that if you're going to change the tag wiki, then it should be changed to reflect the fact that it should not be used during an active tag cleanup and not to disambiguate something, as the excerpts don't serve as enough of a deterrent when tagging a question.

Once the tag is cleaned up, if you can show that it's persistent in coming back and the community believes that it's not a good tag for the site (as you are showing with the algebra tag) then you should have your moderators ask Stack Exchange to blacklist the tag on your site (it's a manual operation and used as a final approach to stem the use of tags).

This is really a response to Martin's answer (which can't be fit in a comment) regarding using the existing tag system to implement this.

I believe this is a very, very bad idea.

First, placing all-caps text in the tag wikis doesn't actually stop the tag from being used. During The great Stack Overflow tag/question cleanup of 2012 (10K users on SO, and I'm the moderator who coordinated that), we placed a disclaimer in the tag wikis of all the tags that were to be cleaned up and not used anymore during the cleanup:

DO NOT USE - Removed as part of "The great Stack Overflow tag/question cleanup of 2012" - See link in Tag Wiki for more information.

And the following was placed in the top of every extended description in the wiki:

DO NOT USE - Removed as part of https://meta.stackexchange.com/questions/128315/the-great-stack-overflow-tag-question-cleanup-of-2012.

---

The reason this was done was because we wanted to stem the use of these tags as they were being removed, not to serve as permanent guideposts for what not to do.

During that time while somewhat effective, the notice at the top didn't keep the tag from being used; we still had questions being tagged with those tags (many have been cleaned up since then).

Even now, on Stack Overflow, with the homework tag officially depreciated, up until recently we were getting a few questions a day, despite it being tagged with featured on meta as well as placed in the community bulletin.

That said, I suggest that if you're going to change the tag wiki, then it should be changed to reflect the fact that it should not be used during an active tag cleanup and not to disambiguate something, as the excerpts don't serve as enough of a deterrent when tagging a question.

Once the tag is cleaned up, if you can show that it's persistent in coming back and the community believes that it's not a good tag for the site (as you are showing with the algebra tag) then you should have your moderators ask Stack Exchange to blacklist the tag on your site (it's a manual operation and used as a final approach to stem the use of tags).