Sounds like they need an outcome less severe than baseball bats.
Electrify the bike?
Superglue them to the bike?
Spray them with fluorescent paint that doesn’t come off?
Ghostbusters slime?
Other ideas...? ;)
Pepper spray the sh-t out of them.
Thin but strong cable attached to somewhere solid about 50-75 ft long.... let them get a good head start before they go flying over the handlebars...
“Other ideas...? ;)”
The internet had one I liked. A cable was attached to the bike and to the post the bike was leaning against. the thief would jump on and haul butt for a few yards and then go flying. A remote controlled electro shock for a fake cell phone “accidently” left behind was also entertaining.
“You steal my phone and put it in your pants pocket?”
thief: “why, no”
bzzzzzzzzzzzzzzt