Table with N associations
A way to resolve this would be to put the joining of things with tags into separate tables.  You would have your original tables users, posts, meta-tags and perhaps items.  In addition, you would have user-tags, post-tags, and perhaps item-tags.  The later three tables would only have the keys from the specific type table (e.g. user_id) and the key from the meta-tags table.

This design would also allow you to have other attributes to a tag that apply across all the base tables.