Is there any way to remove en_* spam-translations?

Unless you have very specific needs (such as an embedded project), I wouldn't worry about the space this takes up. It can be somewhat useful to have more en_* translations, even if they are identical to the original (where that is correct — as you point out that's not always the case here). The reason is that it reduces “false alarms” for untranslated strings.

Bear in mind that the person who is doing this may simply be trying to be helpful, or at least thinking they're earning karma in a way that doesn't cause anyone any problems. That can happen even in cases where it seems very surprising, e.g. because of some misunderstanding or something important they didn't know.

This particular user seems to be from Brazil, a country that speaks Portuguese that differs greatly from the version spoken in Portugal itself. They may simply not have noticed that different versions of English also have differences, because they seem so similar by comparison; or perhaps it's someone overestimating their own language skills. Have you tried contacting them, to point out that some of these translations are incorrect?

If that doesn't work, consider setting stricter translation permissions for your project, and notifying the owners of the Launchpad translation group's en_GB translation team that they have someone on board who's doing work they're not qualified for.