The only truly reliable ways are:
- Buy a Volvo estate and put the bike inside rather than on top. With a 240/740/940 model it is even possible to get a tandem or two inside, fully assembled.
- Leave the car in the street or the drive where it belongs, and store the bikes in the garage. Using a garage just encourages cars to rust, anyway. You may need to buy more bikes to ensure that the garage is properly inaccessible to motor vehicles.