Meaning of Banned

I am sure it must be self explanatory, but I have noticed a few posters with a red "banned" across their icon. Is this put on by the moderator and does it mean that the poster is questionable? I searched and saw a few people mentioning they doubted the advice of anyone banned on here, but I couldn't find a definite reason for banning.

How does a poster get "banned" and how or do they ever get blocked? Just curious.