Honestly just let them die then and they will learn! Sorry but I remember learning how to heal and reading about it when doing Sashtasha and one of the first things always mentioned is how healing causes aggro and only to use Regen once the tank has gained aggro first. Like others mentioned it's possible sometimes that you don't notice and accidentally hit it right at the end of a pull, as I use it sometimes as a sort of half instant heal if I don't need to restore a ton of HP but tetra is on cool down or whatever. It's rare though and usually I just say sorry about that no hard feelings. If someone at level 60 is popping Regen or MedII that often that they are constantly pulling aggro between pulls, then they sound like a really bad healer.

I used to be big on MedII at the start of all pulls (after tank has gained aggro) so that I can DPS a little as well and not worry about others, but I find with the level 60 content there's several times where adds spawn or whatever (like A1 with Faust) where having MedII on just causes all adds to go right to me. Also for most instances there's not really a need to keep MedII up on the DPSs as they don't take enough damage to warrant it. So instead I just use Regen on the tanks and DPS away, keeping Medica II for times when DPS are taking lots of damage, or when doing the actual main boss fight like Oppressor.