In Encounter, regions are one type of placement constraint. Other types are Guides, Fences, and Soft Guides. The terminology is a little inconsistent depending on how you approach the problem (ie, by way of the GUI or by TCL commands or by db access commands) so I can see where this can be a confusing area. So thanks your question- I'm sure others have been stumped by this as well.
If you see a region in the floorplan and you'd like to delete it, you can select it with the mouse (by clicking on it with the left mouse button or via an area selection) -or- with the selectModule command which takes a hierarchical instance name as an argument. Once you've selected all the regions you want to delete, you can delete them by hitting the delete key on the keyboard.
If you want to delete all of the floorplan constraints in the design (regardless of whether they're guides, regions, or fences) you can go to Floorplan->Clear Floorplan->Specified Objects->Floorplan Guides. This is the same as the TCL command "unplaceAllGuides".
If you wanted to delete just the regions using only GUI mechanisms, you could select all of the regions with the Edit->Find/Select Object form by searching/selecting all Modules of Constraint Type "Region" and then clearing them out with Floorplan->Clear Floorplan->Selected.
Or, if you wanted a more TCL-based approach try something like this where we select all of the hinsts that are regions and then delete them:
dbSelectObj [dbGet [dbGet top.fplan.bndrys.type region -p].hinst]
Hope this helps,