Hi Andrea,
What you want to do here is have a "Remove from Flow" campaign triggered off of the flag that you set in SFDC.
How you might do this is...
Create a checkbox field in Salesforce against your Lead and/or Contact record.
Then create a Smart Campaign in Marketo to trigger off of this field being updated. Might look something like this:
Smart List:
Flow:
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If there was more to the original campaign and you JUST wanted them not to receive the email, but to receive all the steps down the line....maybe consider adding them to a list and then you can use "not being a member" of that list in your If Step.
For example:
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Let me know if that makes sense or you have any questions!