I would like to contribute to somehow get rid of a user who is sharing criminal content.
What can I do exactly?

You could tell/message the owner of the room/channel/network that has the user with the illegal content. Owners usually don't want users sharing illegal stuff and will usually ban them from the room/channel/network.

Yes that's very helpful and the Channel Owner should offer a reward of some kind for users time, consideration and thoughtfulness to give the channel owner a heads up on users offering child porn, and underage images of things suggesting sexual actions etc. Owners won't know if those are actually federal, or state agents phishing for pedos or not. Ban such individuals immediately, make it entirely clear that won't be tolerated nor acceptable in your Channel.

DarkMX Staff and Developer,

We Need these additions to the DarkMX and Fopnu applications to help users ward off visitors pretending to Offer child pedo files as well as visitors attempting to share child pedo files. If such individuals ignore the explicit 'Do Not etc.' in the Channel Info file, Channel Owners must have tools which help them deal with this.

1) The DarkMX and Fopnu app needs robust BAN options to help all users ward off such individuals.

2) The DarkMX and Fopnu app needs a file-filter scan that optionally runs thru all visitors file names, searching for Channel Owners filter text words, phrases and alerting the Channel owner if guests are offering files whose name or partial name has been included in the file filter scan.

3) The DarkMX and Fopnu app needs a Autopost timed option. Channel owners could use that for example a repeated or random posting 1x a hour 'Do Not etc.', Special Offering available, Check out the #DarkMX-Fopnu IRC Channel Chatroom on OFTC IRC server', etc.