Which jammer is suitable?
Are there jammers that can only disrupt WLAN on a laptop or phone?
Yes, there are jammers - for WLAN and all cell phone frequencies - wifi jammer. Possession of such equipment is permitted, but initially prohibited - especially in the civilian sector. (don't know what the legal basis is for prisons, police and army)
Courts have already ruled that schools should not use jammers (for example, for Abitur exams).