The RadMap shows map tiles using the standard Silverlight MultiScaleImage control. This way of loading tiles is the standard behavior of that control. And unfortunately we can't change the order of requesting tiles using the TiledMapSource.GetTile method.
There is one way to show tiles for the current zoom level only. You can modify your custom provider for returning correct URIs only for current zoom level of RadMap. In other case it should return null. Also when the zoom level is changed then you should invalidate "null" tiles which were returned for this level from another levels using the MultiScaleTileSource.InvalidateTileLayer method. You can handle the RadMap.ZoomChanged event within implementation of custom TiledMapSource for this purpose.
I have attached a sample solution which implement this functionality for OpenStreet custom provider.
I hope it helps.
the Telerik team