Need idea to stop people from parking in driveway

[Sami Gren]

Hi, 

I have one building that has a driveway for one car on the side and I use it for myself to park since I come very frequently and I need the space for me to park, A lot of people tend to take advantage since they see probably that at night no one is parked and they keep on parking there cars over there and a sign not to paek isn't affective.

Is  there any cheap idea (besides of putting a gate) that can stop a car of rolling into the driveway? 

. Thank you

[Kirk R.]

all cars will be towed.  then get them towed?