Change Request (CRs) represent a form of project waste.
If you are struggling with a constant flow of change requests (CRs) in your implementation project then it is likely that your original requirements definition and the vendor statement of work(s) based on these requirements, were deficient.
Therefore CRs represent project waste because they:
increase project risk
increase resource demand
It is unlikely you will manage to eliminate CRs from your project but the best way to minimize them is to spend more time doing a thorough anaysis of requirements up front, prior to selection.