Plagiarism has been a constant in my teaching experience. AI is a new problem (or solution?) that we are dealing with. These are the strategies that I have used so far.
On the first day of school, ask students to produce a writing sample without technology. You can use this known sample to compare against AI later using GLTR.IO and their new detection software.
However, the above method is overkill. Generally, I ask kids to define words that I don't believe they wrote in their assignments and ask them to redraft until they understand what they turned in.
Address your policy on plagiarism and AI at the beginning of the year and have students test over it. You can use this test later if you encounter blowback.
I have also said that students who are using AI are no longer allowed redrafts of that assignment, but I do that at the end of the year, not the beginning.
Resources
My plagiarism page on my teacher website: https://sites.google.com/view/mccoy23-24/plagiarism
I made a video on AI detection: https://go.screenpal.com/watch/c0Q3XHVCNgd
This "trojan horse" trick to put white text on a white background is glorious: https://www.newsweek.com/teacher-clever-hack-catching-students-using-chatgpt-essay-1893623