I should add that there is one approach that could be taken here. Take this with a huge grain of salt because I am not a lawyer.

When you are posting on Lemmy you are likely granting an implicit license to Lemmy server operators to distribute your work. Basically because you understand that posting a public comment on Lemmy will make it available on your and other Lemmy servers it is assumed that it is ok to do that.

In other words you can’t write a story, post it on Lemmy, then sue every Lemmy instance that federated the comment and made it publicly available. That would be ridiculous.

There is a possible legal argument that twists this implicit grant to include AI training. Maybe you could have a disclaimer that this wasn’t the case. I don’t know how you would need to word this and if it would actually change anything. But I would talk to a lawyer.