You can't really 'turn off' or easily delete custom SFDC fields that have been pulled into Marketo. The easiest thing to do is go into Field Management in Marketo Admin, choose a field you want to get rid of in the right hand tree, then select the Hide field option under Field Actions. This will basically 'deactivate' the field and prevent it from showing up in smart list search.1 of 1 people found this helpful
Adding to Nate's excellent reply:
Selecting "Export Field Names" from the "Fields" tab will allow you to export your Fields en masse and easily keep track of all the SFDC fields you want to manually hide.
The "Field Mapping" tab also has the same functionality.
Curious since this post was so many months ago if you found out anything else besides hiding fields. We left SFDC for NetSuite and it is super frustrating we can't truly deactivate SFDC because of the native integration between SFDC and Marketo. We can't hide any of the Opportunity fields and our Marketo instance has grown so large it is very difficult for us to go back through every form, smart list, smart campaign, etc. that may have used a SFDC field and remove it. I can't believe how difficult this is!