You can do this on an individual basis using the Remove From Flow in the Lead action steps when you find the lead in your Lead Database (in Marketo).
Or if you want to make it available for your sales people using Sales Insight, set up a separate campaign and make it available to them. Here's how it might look:
Trigger Campaign Is Requested, Source is Sales Insight
Remove from Flow, Campaign is [Name of your campaign]
I'd set the availability to run everytime incase a lead gets back in a campaign and they want to remove them again. Also make sure you name this campaign so that it's intuitive for your sales people to know what they are removing somone from.
Hope that helps.
Thank you Jodi, I'll try that.