What is scope creep?
The slow expansion of a project past what was agreed and paid for.
Scope creep rarely arrives as one big request. It is a series of small ones, each individually reasonable, that together add a week of unpaid work.
The defence is written before it starts: deliverables described in countable terms, a stated number of revision rounds, and a price for anything beyond them. Then a new request is a quote rather than a confrontation.
The useful phrase is "happy to. That is outside what we agreed, so it would be £X and add Y days." It is not a refusal, and it puts the decision back with the client.