Are you talking about an advertisement that comes below the posts or the Bitcointalk signature that a user puts in his profile?

The advertisement is common on any platform, you can see on any website, forums, social media platforms, etc. You can put some adblockers to avoid those.

And about the Bitcointalk signature, it is common that most of the users put the signature for promotions. It can be their own website or product or they participate in signature campaigns for promoting other websites. You can ignore those things if you are not interested.