I assume there is a separate vCenter instance in the recovery data center? One way is to not collect data from the back-up vCenter.
One other way is to append the search queries to filter out the back-up VMs somehow. For example, if all of the back-up VMs are tiny (1-2kb) you could try a search query that filters out the tiny VMs.
Is there some other unique characteristics of the placeholder VMs?
I have the same issue and am trying a few things. I think you would need to make the data collection account not have access to the VM placeholders as well as the datastore.