I would recommend sending a PM to CL-Jeremy here on this forum, requesting for the addition of an option to enable/disable aMule in a future Download Master update. I can't do it myself because Download Master isn't part of the firmware, and there are no source code available for it either.
That option would save a lot of troubles - I've seen a couple of users who got bit by it, and probably a lot that don't even realize they are currently sharing their whole download directory over the Emule network.
Personally I think that Download Master needs a major overhaul. The Transmission client it uses is old and lacks support for some modern features like magnets. The openssl library it uses is also old, and potentially has quite a few documented security issues. Part of it has to do with the fact that Asus relies on the old Optware repo from Oleg, which hasn't been maintained in a long time, and uses obsolete versions of various libraries.
So ideally, Download Master should:
1) Either switch to Entware, or stop relying on Oleg's Optware repo and provide every required libraries
2) Update to the latest Transmission version
3) Update to openssl 1.0.1 (and make sure that these ASM optimizations
are included, as the SHA1 optimization will greatly help with hashing performance)
4) Add an option to enable/disable aMule (and have it disabled by default)