Seems to me back in the late 80's - early 90's, Ampeg had a line of amps that had an ignition key you could use to lock your amp. Only reason I remember is the day at University I came home to find my roommate using my gear in an attempt to convince a chick he could play - and having him unplug the cord from the guitar when he heard me, while having the amp cranked to "stun"...
No serious speaker/amp damage - but their expressions and looks of bewilderment were fantastic.
An amp with a locking function would have come in handy...