What are the options?
Several come to mind (spoilers for those unfamiliar with old and not-so-old comics history):
- The Sentinels -- for comics readers of my generation, these were THE anti-mutant bogeymen. They were self-replicating, constantly adapting robots with a mandate to capture and/or kill any mutants. As reflected in the "Days of Future Past" storyline in Uncanny X-men (the basis of the movie), the Sentinels eventually extend their mandate to all superhumans, and then to all humans. These are great for opponents when you've earning some kind of government enmity in your campaign. Or perhaps as a 3rd party foil when trying to capture some metahumans on the grey side of the law.
- The OMAC Project -- in one of the many iterations of the mainstream DC Universe, Batman built a machine called Brother Eye. This machine, meant to watch over the metahuman population (because Batman didn't trust members of the Justice League -- some might say with good reason -- after revelations made during Identity Crisis), was subverted by Maxwell Lord along with some pretty powerful nanotechnology that can turn human sleeper agents into powerful robotic creatures capable of taking out most metahumans.
- Aliens -- no, seriously. From the DC Universe Dominators, who've always shown interest in the meta-gene, to the Kree Supreme Intelligence looking to humanity as an inspiration for the next step in Kree evolution, there's always the potential for some alien race (or set of races, as in DC's Invasion! crossover arc) that decides that Earth is a danger to their continued dominance or existence.
What are your favorite choices or excuses for keeping metahuman numbers in check in comics?