You should reply to the clients when you provide the original quote with a very succinct summary/bullet list of what is being provided - SUPER SUCCINCT. and get them to sing-off on that

So if they come back with anything, and it is not boiled down into the bullet item list of features - its additional service request.

This might help them to further think about their product.

One thing that always pisses me off when getting a quote from a developer is when they say some large number of hours are required for some nebulous task.

"Setup development environment: 40 hours"

or something similar.

shouldn't your development environment be setup? or streamlined such to not require 40 hours?

so, make sure you reply to them with a super succinct understanding of what is to be delivered and this will be the go-to document for any discussion about scope.