Are you shooting RAW or JPG?
When you say it won't let you change it, does that mean there are no lens profiles at all, or no lens profiles that match your lens?
If you set Auto or Default you can select any lens you want from the list of manufacturers and profiles and then the Setup says Custom. Of course this lens may not be the correct lens, but saying you cannot change it suggests there are no manufacturers or lenses listed on the dropdowns.
Most lens profiles are for RAW only, so if you shoot a mixture of RAW and JPG then for the JPGs the lens profile may not exist. If you are enabling Lens Profiles and have Auto or Default set and it chooses the wrong one then it may just be choosing the top one in the list for Canon.
If you are shooting raw and the profile does exist in the list then you can select it, then choose Setup: Save New Lens Profile Defaults, and select Default from the Setup instead of Auto and it should pick the correct lens every time. Assuming it does do that, then you can set Use Lens Profile = checked and Setup: Default and set those as your new Lightroom Defaults and it should pick the right lens.
If you are shooting JPG then the lens profile may well not exist and there's nothing you can do about that except not shoot JPG. If you feel like it you can hack a raw profile to be a jpg profile and put that hacked copy in the user-lens-profile area and LR will see it.
