Depending on what you want to do, you could use either a validation rule, sharing rules, or roles (if you auto-assign the account to have the CSM as the owner).
Roles and sharing rules are two kinds of record-level access control that Salesforce provides. Profiles and permission sets grant access to particular kinds of things (access to accounts in general, access to particular fields on an account, etc.) Roles and sharing rules allow you to determine access on the basis of record-level criteria.
As TSmith commented, a sharing rule is probably what you're looking for. They allow you to prevent read access (CSM can't see the record in Salesforce) or prevent edit access (CSM can't edit the record).
A validation rule would also allow you to prevent edit access, but the user experience is less desirable, since the user doesn't get blocked until after making edits to the record and saving. Sharing control prevents them from even trying. A validation rule would be appropriate if you wanted to allow the CSM to edit some fields on account, but not all. Picklists can be used in formulas; they just require a special syntax:
The down-side with sharing rules is that you'll need to change account access to private, then extend it out based on criteria or roles. Your users may lose access to things they could previously see, and should see, until any kinks are worked out of your sharing rules.