I'm asking this generally because I assume that the offending plugin is using some part of the WordPress API to prefix the output from the other. I'd like to either eliminate what's creating the input or disable the ability to create it. I'm fine with hacking one module or the other but I can't touch core.
Any ideas out there? Is there an add_stuff_to_outgoing_feed hook/action I'm not familiar with? Can I disable it on the plugin creating the feed?