Not mentioning any names, but I've also had my fair share of people KS'ing my thanatos. People come in teams, so guessing it's element has absolutely nothing to do with KS'ing it, any idiot can read damage numbers.
I really like the idea of disabling pvp for that reason, but also possibly once a single person starts the summoning, nobody else can interrupt it, as I have had other people come and also put fragments in while I'm doing the same.
I am in full agreement on disabling pvp for thana_boss.