I thought this was a good question and went to try and find out. I grabbed the old source from https://ycombinator.com/arc/arc3.tar and started grepping around, but couldn't find any function to delete comments (only links, which do delete, but don't get purged from the cache it seems). I didn't set up Arc to try and run the thing though.

If anyone has more knowledge, or understands the source better, or has newer code, I'd be interested to know.