Tape this to your monitor:
"We'll be more than happy to make the changes that you've noted to this most recent round of [name of deliverable]. However, to accommodate your request, there will be an impact on the project schedule and overall scope, which may result in a change order. We'll get back to you within [X number of hours] with an idea of what kind of impact, if any, these changes may have."
Painting a clear picture of what deliverables and edits are included as part of your overall service offering, and then being just as clear about what client-desired changes are outside the boundary of that offering, are a fundamental attribute of running any service-oriented business.
Many designers are afraid of bringing up that a change in project scope—from an extra round of creative tweaks, to additional features added to a website—will cost them their client relationship. Designers dread having to charge for an extra round of changes when a client says that their design work didn't cut it, even after an excessive number of edits.
Well, I'm here to tell you that in those situations, there is a shared responsibility between you and your client to negotiate the appropriate outcome—to make both parties satisfied with the results of your design efforts. If you spend all of your time working to please the client, you may be delivering exceptional client service, but it may not be in your best interest as a business professional. And in those situations, it is completely within your rights to read what I'd noted above to make both parties aware that you are verging outside the boundaries of your existing agreement.
Of course, none of this will help if your client had signed off on a scope of work that didn't delineate the amount of time, discrete deliverables, or quantity of changes necessary to complete a project in an effective manner. And you must always include in your client agreement that additional changes beyond scope may result in an additional fee (by dollars per hour or by round of deliverable edits), allowed by client approval in writing.
If you haven't done that… then you won't have any recourse if either party is dissatisfied with the outcome of a project. (After drowning in dozens of rounds of changes.)
And while you might be tempted to blame it on your client, it's actually your fault.