I want to import a list of contacts into Marketo from a csv excel file for a one time email campaign. I do not want these contacts to sync into salesforce. How do I prevent that?
By default, they will not...but
There are several related threads if you do a search.
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.