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There are many inactive user profiles on SO that were clearly registered by spammers. The spam posts (if there were any) are already removed but the profiles remain. Some of them put their spam text into the "about me" field, see for example
deleted and deleted. If you google this text, it has been spammed all over the web. I guess that the purpose is manipulating search engines - when searching for particular keywords a website with a link to the spammer's site will likely come up as a result.
Is that something that we need to worry about? I can see why the answer would be "no": unlike spam in questions, this isn't a problem for SO - nobody sees these profiles. So this isn't really worth investing effort into. Then again, I hate spam and don't like making spammer's life easy.
If the answer to the previous question is "yes", what can be done about it? Obviously, anything requiring much effort from the community or the devs isn't going to work. One solution I can think of: stop displaying "about me" text for profiles that have neither answers nor questions and that have been inactive for at least a month. Maybe stop displaying the webpage link as well.
Edit: I found many dozens of user profiles (now all deleted) when searching the user database by the keywords "sms", "market", "website", "free", "cheap", "gift", "flower" and "florist". If you then sort by all time reputation score and concentrate on accounts with reputation 1 to 11 it isn't hard to find lots of spammer accounts. (Some of them have the necessary keywords in their user name, they don't use the "about me" text then.) Still, combing through the user database like this requires far too much effort and I definitely don't think that it is a viable solution.
As staticbeast notes in the comments, the Data Explorer is great for finding these profiles.