Janet,
Is there a way to prevent people from deleting [their own] posts?
As J said, no.
I'm on the opposite camp though. I support users having the unfettered ability to remove their own contributions across the board. I'm not sure how Groups.io ended up with a mixed situation, granting the ability through the UI but not the contract right in the TOS.
I would prefer that the Groups.io Terms Of Service document be amended to make such removal a contract right. That is, I would replace the grant of a perpetual and irrevocable license with a limited one; a license which terminates when the member removes his/her content from the service.
I don't expect this to be a high priority issue for Groups.io right now. And it has been a recurring issue in Yahoo Groups over the years, but firmly settled in the members' favor by the Yahoo TOS (paragraph 9.1).
Shal
Shal Farley