Yes, you can. Before you drive off, and while you still are on a wifi-connection, carefully go through every single tile on your route. Waze will not need to pickup those tiles when you are driving, it will only get live traffic data. However, this is a tiresome routine for a longer route, and Waze does not use all free memory for maptiles, so at some point you push the oldest tiles out of the memory. Apart from that: on the iPhone you can switch on datacompression and switch off downloading of trafficdata - not sure about the versions for other equipment.
AM for some 25.000 km² around Maastricht, the Netherlands
iPhone 3Gs / iOS 6.0.1