I just tried it, and I was able to delete maps. I went to "Maps > Manage Maps...", selected "Delete Maps", checked the Zoom tab and the Area tab to make sure they corresponded to what I saw on the screen, and tapped OK, and after a pause saw the pretty maps I'd been looking at replaced with scaled-up maps from the next zoom level out. Scrolling around shows that only the maps that overlapped the screen (i.e., the maps covering the rectangle specified in the "Area" tab) were deleted -- outside that rectangle I still have maps at the current zoom level.

However, I got a little pop-up dialogue showing that the Nokia was connecting to my local network at the start of that process. That shouldn't be necessary in order to delete maps, but I'm guessing map deletion reuses a loop coded for map download, and connecting to the net is the first thing it does. It might be that if you can't automatically connect to the net, the deletion is being aborted. So try connecting to the net first before you delete maps.What are you doing in order to try to display Level 10? What is your current zoom level?

Maemo Mapper will scale up a higher zoom level to display a lower one -- for instance, if you're viewing zoom level 7, and you don't have maps at level 7 but you do at level 8, it will double the size of the level 8 maps and use them for level 7. But it won't do the reverse: if you're viewing zoom level 7, and you don't have maps at any higher zoom level, but you do have maps at level 6, Maemo Mapper won't shrink the level 6 map images to display level 7; you'll just see a black screen for areas you don't have map data for.