If you simply connect the batt and the solenoid kicks in (they dont spin themselves - the starter motor does) or the starter spins without kicking into the ring gear or both happens the starter circuit is "on" or closed.
The most likely place is the starter relay which should be in your "relay box" if its a late model car. It will be a 4 or 5 pin relay.
The starter itself gets power direct from batt but the triggering is via relay only not direct. They can be intermittent.
If its an older model there is still a relay but it may be incorporated in the dash wiring or on the starter itself or on the firewall. Old models can have a separate solenoid altogether.
You dont say if the starter runs when the ignition key is in the off position, on, or start etc. Does the key do anything as far as starting the engine or is the starter just playing all by itself?
Well done SA pollies (for once). Hoon laws are long overdue to protect car enthusiasts against idiot drivers.