Tenants Using all Visitor Parking Spots (Vic)

I live in Melbourne in an apartment  block with 25 lots. We only have 6 visitor  parking  spots which are continually taken up by tenants renting 1  bedroom apartments  but have  two vehicles. We have placed signs saying  “Visitor Carpark Only, maximim 4 hours or your vehicle will be towed” which I was against as the tenants called our bluff and continue to use our visitor spaces at will. What options does our owners corporation deal with this?