can I restrict the values of a list in a client script? My other option is to just return an error but I would rather them just not have the option to pick a value in the first place

You can create a dynamic custom field in script and populate that field with whatever values you wish, then persist to the existing field. I would hide the native field. You can do this in a mixture of before load and client scripts. You can also use insertfield to place the dynamic custom field where the native field was