One way to minimise their PR efforts is to create significant Streisand effects on their work. But some PR companies are so desperate that they would probably even be delighted with that.
Part of the reason there isn't much of a Streisand effect here usually is that in the common case, honestly nobody cares about these articles. A PR company writes an obvious fluff piece about some obscure internet portal or logistics company or for-profit university. If someone on Wikipedia catches it, they might try to tone it down or even delete it. But most of the time: 1) nobody even sees these articles; and 2) it's barely really worth the effort.
If a PR company tries to fluff up a politician's article who's engaged in a high-profile race, or some energy-industry PR people edit an article relating to climate change, or the Turkish government hires a PR firm to edit the page on the Armenian genocide, then people will notice and care. But nobody reads the thousands of articles on obscure companies. The minus side is that they're usually crap articles, but the plus side is that they are relatively uninfluential crap: all they tell you is that some small company exists and thinks it's great, and they get a handful of views from Google. Usually they are not even really linked from other parts of Wikipedia: when the PR companies try to insert spammy links from other articles that people do read, that's when these fluff articles are caught.