Awww, sorry man. That really hurts. In Edinburgh, we have same problem in last 3 years - kids stealing old bikes rather than fancy stuff, just to ride around on and crash. Previously, we thought no resale value = won't get nicked, but no longer true.
No garage? Ground anchor in your drive. Cover it. They've been twice, they will try again, your insurance budget and stress levels can't take another hit.
Good luck, and once again, really really sorry. Scumbags.