I have a Mikrotik Router with a wireless interface.

All useres connect to the wireless hotspot. For using the internet, they are required to establish a pppoe connection with a pppoe server in my network.

In order to keep useres from doing someting else then connecting with pppoe and surfing the web, I need some firewall rules.
e.g.: it shall not be possible for the clients to connect to the wireless station and ping network equipment or transfer data with other clients.

I am looking for some general rule, like: "drop all traffic comming in from the wireless interface except the pppoE connections"

just do not assign address to the wireless interface