Some spammers are getting into the habit of posting almost useful answers in order to disguise their spam.
This is a difficult call though. 99% of the time these types of answer are spam but occasionally it is someone just trying to be helpful while getting a little exposure. The difference between the two is that a spammer tends to almost hide the link amongst their text in order to trick you into clicking it.
For the few cases where people are self promoting in a signature it would be worth editing out the signature and pointing them at our various policies.
Behaviour, regarding signatures
Do not use signature, taglines, or greetings.
Every post you make is already “signed” with your standard user card, which links directly back to your user page. If you use an additional signature or tagline, it will be removed to reduce noise in the questions and answers.
Your user page belongs to you — fill it with information about your interests, links to stuff you’ve worked on, or whatever else you like!
Don't talk about your product / website / book / job too much. Folks will read your answers for their ability to solve a specific problem; if you're good at doing that, then they'll find themselves more interested in who you are and what you're working on.
If it is a one off and seems like an honest mistake or misunderstanding how we work then feel free to edit out the offending signature and point them to our help centre. If they persist, or you just don't want to get involved, then you can flag it for a moderator to have "the talk" with them. Either way is fine, but we encourage the community to be a part of the moderation system and that includes setting new users on the right path. If they ignore you or roll back your edits then get a moderator involved.
If it looks like they're just here to get that link posted and disguising it using an almost helpful answer then flag it as spam.