If I post something illegal, who bans me?

While it's impossible to be banned from the blockchain, individual interfaces might choose to hide your content. There are multiple interfaces to the Steem blockchain however, such as Steemit, SteemPeak (I'm using SteemPeak right now actually), and Busy. Even if an individual interface censors content (which some have done in the past, due to legal reasons), other interfaces will still show the content.

who is the one controlling all the algorithms and coding for the website?

Same for the coding of the website: each interface has it's own coding. Steemit is coded mostly by Steemit Inc., SteemPeak is coded by the SteemPeak team, and Busy is coded by volunteers mostly.