Constant private use of visitors parking.

One of the cars has been parked in the visitor parking space for about over a year which left us with 2 spaces for about 4/5 houses to be able to have space for visitors to park. Few weeks ago the same owner parked a second vehicle behind the first one and has left it there permanently leaving one space for visitors to park. The owner has his own allocated parking space just like everyone else but chooses to use the visitor parking space on the top of his own leaving many houses without any visitors parking space. Please advise what can be done as it seems soon we will have no space left at all.