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No. He's putting notices on people's windscreens asking them not to park there because the garage space belongs to him (which may or may not be true).
But the thing is this: he/she does not use the garage to store a vehicle. The landlord stores junk in the garage and there is no room for a car. In addition, I have never saw anyone other than the landlord access the garage.
My view is that if the garage space is to be used to store a vehicle then of course, the space in fron of the garage space should be left vacant. I have no problem with doing that.
I have a problem where someone claims to rent a garage, but does not use it or show any signs of using it, but believes that by renting a house (and perhaps a garage space), it also gives him/her preferential parking rights on what is essentially a public highway.