To free the assigned drive-letter again use the subst command like this:
Subst x: C:\Very\Long\Folder\Path
Note when restarting the computer then all mapped drive letters are forgotten. One can put the subst command into a startup-script which is launched at startup, so the mapping is done automatically again at user login.
Subst x: /D
Note the utility Visual Subst is a more GUI friendly utility for managing virtual drives created with subst (Has option to mount drives at boot / startup).
Note Windows Vista User Account Protection introduces a special behavior with Subst.exe as the drive-letter is only seen by the user-account that has performed the mapping. This means that when changing between regular user and elevated user then, one has to perform the subst command in both contexts.