How do I prevent a specific user from accessing a shared printer using regedit?

You'll need to modify the permissions on the printer's share name within `HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanmanServer\Shares`. Find the share name corresponding to the printer. Then, you'll need to use the `Security` tab (accessed through the share properties in Computer Management, not directly through regedit) to remove the user's access rights. Directly editing the security descriptors in regedit is complex and not recommended unless you are very familiar with the format.