Hi Chris F., you can pause the imports happening on your table by clicking the "stop updates" button in the bottom right of your workspace (which appears when updates are taking place in your table -- like a salesforce import). Attaching a screenshot below to help :)
Will have to ask the team, I really don't think we have that option but will ask if it could be added. Otherwise, not sure if there's something you could adjust in SFDC to prevent new leads going to that list (if it's a list you're importing from of course)
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