Just found this: Pay to delete? - #20 by janitor
So it seems deletion of data is not possible, at least not yet?
I’d say the space it takes is more or less neglectable, but as it has to be duplicated whenever a node containing such stuff has network issues, what comes to my mind is a network that is constantly busy duplicating content it already has, which gets worse the more content is available in the network. Or am I over-estimating that effect?