Don't allow any wikipedia content, only links to articles. The main content of the pages should be about how the tag is used on stack overflow. Require reviewers to provide a source of plagiarism and if the edit is approved, it should be flagged for further review. You should give a badge to the person who found the plagiarism.
The tag wiki's are suppose to be about how the subject/tag relates to stack overflow and not an encyclopedia article. No one will plagiarize an article on how a topic/tag relates to stack overflow because such an article is unlikely to exist. People shouldn't approve wikipedia content as it really isn't helpful to see what kind of Q/A belongs with a tag. If people only approved by this guide line, then I don't think that most plagiarism would exist.
A single reviewersingle reviewer might find plagiarism, but the current system will not act on it. This is fairly intelligent as the reviewer isn't required to provide any source that they believe was infringed.
People also learn by example. The fact that we have so many tag pages with wikipedia content makes it seem that this is okay. In fact, if wikipedia is cited as a source, it is okay; just not without attribution by my understanding. Changing it so there was no wikipedia (or other attributed content) would give no people examples of this as being okay (and missing the fact it was attributed and this is important).
