Just report and block. Anybody can look at LFG members and message them without having to be in the fireteam themselves. The likely scenario is that these persons assume that a percentage of players who join LFG’s may need help and therefore may consider the spam as a legitimate option. I had this happen last week. I’ve not had two at the same time like yourself though, but that could just be pure coincidence - perhaps these spammers have peak hours.