If you need to remove a specific set of leads from Salesforce but leave them as people in Marketo, you can use the Delete Person from SFDC flow action.
Only available when integrated with Salesforce.
In the Database, click the person you want to remove from Salesforce. Then click Person Actions and select Salesforce.
Select Delete Person from SFDC.
Be sure the Delete in Marketo setting is false, then click Run Now.
After the flow step runs, your person will no longer be a lead in Salesforce but will remain in Marketo.
If you set Delete in Marketo to true and delete the people from Marketo and the leads from Salesforce, they are gone forever. This can’t be undone.