If you want to prevent some SFDC leads from syncing to Marketo,
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create a custom field called "Sync to Marketo"
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submit a support case to Marketo, and ask them to set up the integration on the backend to only sync leads where that field is checked
You can then either manually uncheck the box for leads you don't want to sync to Marketo, or create a formula field or workflow in SFDC, or a campaign in Marketo, to manage whether the box is checked or unchecked.