Out of the loop

It's a system for publishing your posts and comments. That is literally what it is for: making them public. When you post, you thereby willingly surrender control over whose computer your message might be copied to.

The Lemmy server software, in particular, defaults to open federation. This means that the "normal" mode of operation is literally to permit absolutely anyone anywhere in the world to read (or otherwise process) whatever you're posting.

So you can't reasonably expect to prevent or deter someone from using the material that you have caused to be delivered onto their own hard drive.

It would be interesting to require as a condition of federation that all posted content be placed under a share-alike license like CC BY-SA: you may do whatever you like with it, but you must cite your sources, and the work you create thereby must also be reshareable under the same terms.