I know you can hide fields, but I would like to permanently clean up our instance so that we don't have confusing lead fields in our system. This has been a huge frustration of mine.
Jon Blumenfeld, you may want to follow up with Marketo support and see what they say. Anything I've ever heard is that deletes aren't possible though, only hiding.
Hiding the field should effectively remove it from view in the interface though, so it doesn't appear in lookups etc. Where are you running into problems after hiding?