Hi Alexa,
You're correct - the "Lead is Created" trigger triggers off of any lead that is created in Marketo for the first time - no matter where they came from - as long as they are not already in Marketo. If you wanted to specify a specific source, you can select that from the "Add Constraint" dropdown.
For the campaign you mentioned, you can use the trigger, as well as a filter for company names/email addresses that you want to remove. Instead of removing them completely, they could also be marked as Marketing Suspended - just another idea.