TO CLEAR your CUSTOM MAPS
( from Windows Start Menu) , enter :
C:\Program Files (x86)\Steam\steamapps\common\Team Fortress 2\tf\download\maps
in the search window
- That takes you to your custom maps folder in Steam.
- Delete all of them that contains the map name, i.e.. ; Teien, or Waterfront.
- RESTART Steam

If your games are installed on another drive, go to (example):

D:\Games\steamapps\common\Team Fortress 2\tf\download\maps