Remove the 'tag: latest' part from the appropriate entries in the externals block. I do that only for the Ace3 libs, for precisely the same reason. Here's a verbatim chunk out of one of my .pkgmetas:
# for Ace3 stuff, it's usually better to grab the alphas (they get tested)
# than waiting for bugfixes to get tagged
Once there's a release tag again for MoP, it's probably wise to switch back to 'tag: latest', but for now these alphas are better tested than most other libraries' full releases.