Unfortunately I don't believe artists have any current legal recourse for this and, honestly, I'm not sure they require one.
Legal Eagle has a great review of common legal questions with court judgements on some. The bullet points are that an artist's style isn't protected by copyright and training and AI on an artist's work can be considered fair use.
Asking an AI to draw art that looks like it was done by Greg Rutkowski is no different than asking a human artist to do the same thing. The human artist would look at Rutkowski's works and do their best to copy the style of it. As long as the artist doesn't recreate an identical looking piece, it's not copyright infringement.
If artists could copyright their style it would stifle creativity. Same thing if artists could prevent their work from being used as instructional or reference materials.