Trying to remove old leads from former lead owners.

I am trying to remove old leads from Marketo that are owned by former reps at our company. I pulled together a smart list that looks like - Lead Owner Last Name is 'Person X, Person Y, Person Z'. After looking at the results, I noticed leads and contacts are being brought in owned by current reps within our named accounts. I tried every filter in the system to possibly stop this. Marketo is pulling in leads that were created by the former reps I am trying to isolate, but the lead or contact is assigned to one of our new reps and we don't want those records to be deleted. How can I bypass this? Thanks