If you right-click-and-drag in the tilemap to select multiple tiles, you should be able to press Delete or choose Edit→Clear Selection (disabled when it can't be used) to erase the selected area. It would become all tile $0:00, palette 0, no X/Y flip. Likewise enable controls for X-Flip Selection and Y-Flip Selection if the format supports flipping.
If you right-click-and-drag in the tilemap to select multiple tiles, you should be able to press Delete or choose Edit→Clear Selection (disabled when it can't be used) to erase the selected area. It would become all tile $0:00, palette 0, no X/Y flip. Likewise enable controls for X-Flip Selection and Y-Flip Selection if the format supports flipping.