we accidentally synced leads to the wrong program. They are now in the right one and we removed them from the program. However, the incorrect program is still showing up in interesting moments. Is there a way to remove that?
There is not. Interesting Moments are not deleteable (just like anything else in the Activity Log, actually!)
There's an idea here for Marketo to add a flow step of sorts for marketers to remove/delete interesting moments logged previously for a person. As Sandy said, you wouldn't be able to remove the particular IM creation activity from the person's act log, rather if Marketo in the future adds a flow step to delete an IM, you'd be able to delete an IM for a person and the same should ideally sync over to the SFDC removing IM from the MSI.
But off the bat, I think it'd be difficult to reference the right interesting moment to remove from a person in the campaign flow, assuming that there would be more than one of them for most people, and especially if there are any lead-specific items in the IM description as well.