To execute any command as root, do
su -c 'your-command-here'
For example, to remove all files in the directory /home/user/cruft, do
su -c 'rm -rf /home/user/cruft'
Enter the root password when prompted.
As mentioned above, be *EXTREMELY* careful. If you accidentally insert a space after /home in the above command your entire /home directory will be hosed. Fscked. Gone. Irretrievable.