Yep - you will need to have some admin rights though. Basically if you have a "Marketo" dedicated SFDC account, you restrict what leads/contacts that account can see. Then, when you turn on the sync with Marketo, only the leads that meet the criteria you set in SFDC will be passed over. It is a little bit tricky, but works when you get it set up properly.
Here is an article on sharing rules in Salesforce
https://help.salesforce.com/HTViewHelpDoc?id=security_about_sharing_rules.htm&language=en_US
Taylor