Restrict users from changing the account name on the Opportunities

Question

What is the best way to restrict users from changing the account name on the Opportunities?

Tried making the 'Account' field a read-only field but unfortunately if the Sales Person creates an Opportunity from scratch (not converted with leads or from an existing account) they are stuck as they can't add a company name in!

Ideally the option above is the simplest to do but can you use permission sets or validation rules that allow exceptions (i.e. make it a read-only field only when created?)

Answer 1

You can easily do this using validation rule.

NOT(ISNEW()) && ISCHANGED(AccountId)

And throw error,

You can't change Account.