Kristen Malkovich, when you delete the invalid leads, do you only delete them from Marketo or also from your CRM system? I'm getting a lot of push-back about deleting leads in SFDC (due to losing history) so trying to figure out the best solution.
Always defer to your SFDC admin. If they don't want to delete them out (and you're cool with losing history in Marketo, which you should be!), then go ahead and just delete them in Marketo.
Kristen, I'm looking to delete leads from my instance now. However, I'm concerned that if I delete them in Marketo, they will only resync from our CRM since they have not been deleted over there. Any idea on how to prevent this from happening?
You could also specify to have Marketo delete the leads from your CRM as they delete from Marketo.
However if you don't want the leads to be deleted from your CRM (deleted from Marketo only), then as suggested above you'll need to make sure your CRM can be set up to "not see" the leads that are being deleted.
CRM Sync - Marketo Docs - Product Docs would be a good place to start, and you may need to contact Support if you end up needing to modify your CRM field mapping or permissions.