how can I avoid 50 minutes trial classes?

Hello, I’d like to know if anyone has found an effective way to avoid 50-minute trial classes. I put a recommendation at the top of my profile to choose only 25-minute trials, but many students still select the 50-minute option. I suspect some students book a cheaper class and then try a more expensive tutor just to get a free lesson. I don’t want to block everyone who requests 50-minute trials because I’m worried Preply might ban me or something. Has anyone tried putting this in their profile title? Did it work?