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Long story short, create a "Do Not Sync to Marketo" field, populate this as true, and then delete these lead from Marketo. When you run the flow to delete, make sure you choose false for "Remove from CRM" to keep these contacts in SFDC.
That's a good start, but not enough. You need to have a sync filter set up set up, with a "Sync with Marketo" field. You need to have sync with SFDC before deletion, so that the true value of the "Do Not Sync to Marketo" field flows to SFDC. Then you need a process to flag the field "sync with Marketo" as false.
You need all this, otherwise the record will come back to Marketo as a new lead each time a change is made to it in SFDC.
Read "Sync with Marketo" mysteries part 2 too