The way i found to clean all files and subdirectories on a directory is to remove all files recursively with del as others already did, then use a for to remove each subdirectory:
del /s /q "C:\Path\to\directory\*"
for /d %a in ("C:\Path\to\directory\*") do rmdir /s /q "%a"
Tested on Windows 10, I think it will work on Windows 7.
If used inside a bat file, the % needs to be escaped and '%a' becomes '%%a':
REM Inside a .bat file:
del /s /q "C:\Path\to\directory\*"
for /d %%a in ("C:\Path\to\directory\*") do rmdir /s /q "%%a"