How do you prevent leads imported into marketo from syncing to salesforce?

By default, they will not...but

• how does your lead lifecycle or MQL sync work? Will these leads get pushed?
  • in this case, make sure to exclude this list from the sync
• do not add these people to a Program that is synced to SFDC.

There are several related threads if you do a search.

Chris,

Go to Marketing Activities and look on the right of the screen. You should find the Campaign Inspector. If you do not see it, you can activate it from the Treasure Chest in the Admin Panel. In the Campaign Inspector, search for "sync" in the quick find at the bottom of the page. That will help you identify any sync point to SFDC. Hopefully you only have one central sync point (if you have one). If not, you will have to ensure the list will not meet any qualifying criteria for any of the sync campaigns. If the list is small and won't take long to upload, you could turn the sync campaign(s) off until you are done uploading, or, like @Josh Hill said, filter the list out.