You should be able to, you would want to start by adjusting the campaigns that actually fire the email send for that email, if you remove the email first, it can cause the campaign to fail, but remove it first then you should be ok. Unless I'm missing a something unique about the campaign.
Marketo will not actually allow you to delete an asset that is currently being used or referenced by another asset or Smart Campaign. In order to delete the email(if that's what you really want to do), you must first remove it from all flow steps, triggers, and filters that it is associated with.
The first step in ensuring that your leads don't receive that email is removing it from the flow steps associated with it, and then if you want to delete or unapprove that particular email, to remove it from any filters or trigger that reference it. Hope that helps you out.