I've yet to see a non-trolling healer use Medica 2 pre pull, and pre pull regen usually happens when it is mistimed and used towards the end of a trash smash. Obviously, we all have different experiences, but generally speaking both tank and healer pretty much have to deliberately work against each other for agro to become a mess. If your healer is spamming their AoE heal when the dmg doesn't warrant it, you are likely being trolled. Boot this healer if they don't listen to reason.

Pre pull regen is also actually a good thing on bosses as tanks typically open with an enmity skill, and its auto attacks start getting up healed immediately and can spare them using a CD at the start when bosses just love to follow up an auto attack with a skill.

Furthermore, every healer has a threshold, and this is not set in stone. Some won't start healing until health is below 50%, others like to keep their party near full HP, especially if they don't know what's coming. Gauge how much it is actually a problem if they are over healing, and choose your battles. If putting up a fuss is counter-productive, then it might be best to just let it go.