@Stuart This depends on where the data will be published
Your PowerBi environment - meaning one you can log into at PowerBi.com (and you can limit which workspaces your client sees)
- you can separate the report from the dataset - https://www.youtube.com/results?search_query=powerbi+separate+dataset+and+report
- once this is done, you can publish the dataset to a completely different workspace from the workspace of the report - and don’t give your client access to that workspace
- even if the client downloads the report, they won’t have access to the dataset with your measures and structure.
NOTE: if by ‘hand over’ to your client, in order for them to continue to design the report, someone at their end will need to have some level of access to the dataset in order to do effective report design.
Your client’s PowerBi environment -
you have no control over, and the dataset needs to be available for the report to be of any use to the client