You’d need to check your Development Approval and how many visitor parking spaces were specified.
Once you lease them for private use, they have ceased to be visitor spaces and you leave yourselves wide open to complaints for anyone, for instance, who required daily care and needed parking.
You would then face problems with the person who has been illegally leased a space that is no longer available. And it would only take one call to council for them to get involved too.
Lockable bollards are incredibly effective way of controlling car parking – and they are not that expensive.