Puma spare wheel

My girlfriend recently had her spare wheel stolen from under the car. Is there anyway we can stop this happening again, i.e - purchasing some kind of lock?
Also could you clarify what type of wheel the spare would be? I.E - a full size tyre or a smaller one. Also would it be just a steel wheel or alloy wheel as the rest of the wheels on the car? She thinks it is an alloy but I told her that most spares are just steel wheels.
Many thanks for your help.

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