Nuisance calls from a specific person may be one of the few good reasons to change a phone number. VM will do one number change FOC.
Worth noting though that any 'new' number will actually be recycled from VM's pool of allocated numbers. There is a possibility that it may come with its own set of problems e.g. if it has been used in the past for a business etc. or has its own history of nuisance calls.
Some phones have the feature to block specific individual numbers but may be a bit limited in their effectiveness. Call screening devices such as trueCall can be configured to screen or block calls in a highly specific way.
One of the VM forum team will respond on here within a few days to assist further.