well the solution for Arch could be simple (but also easily worked around by Manjaro) -
simply block pamac as user agent 
but Manjaro could work around that by changing the user agent of pamac to something generic and the block would not work anymore. But changing the user agent to something generic could also make other stuff more complicated: https://gitlab.manjaro.org/applications/pamac/-/issues/709
Edit: Apparently, pamac was already blocked in the past: [Resolved]Pamac currently blocked from the AUR
Anyway, the long term solution for too much traffic would be to implement a good CDN for the AUR webserver to mitigate the requests and traffic that go through to the origin - after all, most stuff could be cached by CDN edges because it does not change often.