A regulation in the EU grants its citizens a right to erase personal data. How that would apply to groups.io, I don¡¯t know, but personally, I rather wish it existed in the US. I¡¯ve shot my mouth off a number of times and I wish I could make the network forget.
No, there's no way to prevent them from deleting their posts. I have lobbied for this as well. Others argue that members are legally entitled to remove their own content. I think the legal argument actually goes the other way, because users grant non-exclusive license to display their content in any way, shape or form. The current TOU does not require to allow users to remove their posts, and I feel strongly that a group should be allowed to disable that ability. There are threads on beta about this. You might want to have a look.