Is there any effective way of keeping weeds down on a monoblocked driveway?

An effective way but not permanent is to use a quality weed killer, and a lot of time scraping out the weeds and debris, unfortunately nature wins, also depends on the installation of the patio and monoblock driveway, whether they used good membrane or not. We can't really stop them from growing on top of the drive or patio but only to reduce regrowth. Hope this is helpful Jamie

Yes powerwash it but do not put any fine sand down then that way you will not get any sand weeds