Chances are, while you suppose that your project manager has a 'pernicious love of sneaking in "scope creep" when they can', his perspective is probably different. It is probably more productive to make sure you understand his point of view.
This is of course in addition to communicating your own point of view, namely the consequences of the scope creep. You probably need to explicitly identify the additional work in your development plan, estimate how long the additional work will take, and explain that this means a delay or dropping some other feature.
This only works if your estimates are any good. This does not work if you do the additional work off the books, so that the additional time spent is not visible. Otherwise, you might meet the deadline for other reasons, such as harder work or efficiency gains, and your project manager will just remember that scope creep did not affect the delivery date 'last time'.