First, burninate the tag, if there are currently questions tagged with it. If there aren't currently questions tagged with it, you probably don't need to have it blocked - but see Step #2.
Once the tag is gone - no longer used on any questions - decide if it needs to be blocked from ever being used again. Some tags, once removed, never come back: typos and ambiguous tags that are replaced with similar but more specific tags rarely need to be explicitly blocked. Others, are more alluring: tags that can be found in the name or primary topic for the site often have this problem. If you find yourself repeatedly removing the same bad tags, proceed to step #3.
Start a new discussion on your site's per-site meta (don't re-use your burninate discussion question). Make sure there's clear agreement among the folks in your community as to why the tag should be disallowed. The better you're able to explain it, the easier every step that follows will become. The tags you should use for this discussion should include discussion and some tag that indicates you are requesting a tag block. A good tag to make (if your Meta doesn't have one) could be tag-block-request, but some metas have a different tag (MSO uses blocklist-request). If you already posted a meta discussion during Step #1's "burninate" process, you'll have this rationale readily available.
As part of the discussion above, work out the warning text for this tag, including any suggestions for alternative tag options. These warnings can include some Markdown including links, bold/italics, and bullets. That said, keep it as simple as reasonably possible. As an example, here's a more complex block warning for the passport tag on Stack Overflow:
Once any and all issues raised in your meta discussion have been resolved, flag it for moderator attention and link to this answer. The moderator team will review the matter and, if all requirements have been met, tag the question as status-review to escalate the request to the team here at Stack Exchange. However, you should make double-certain that all steps outlined here have been completed before you do - in particular, if we lack assurance that a given tag is actually likely to return, or if no clear guidance as to why it is inappropriate is available, we're unlikely to honor the request - remember, there's a good chance we know nothing about the topic of your site.