If you can't launch the MMC because of restrictions (Did you apply this GPO to the whole domain or something?!) then you can use PowerShell's Group Policy module.
Log in as a Domain Admin and launch PowerShell on a computer with the AD Commandline Tools from RSAT installed . Run:
Remove-GPLink -Name "GPO That Breaks Everything" -Target "ou=OU Where It Is Linked,dc=contoso,dc=com"
This will remove the link to the GPO without destroying the whole GPO.
If you want to just remove the whole GPO all together, then you can run:
Remove-GPO -Name "GPO That Breaks Everything"