It's getting more complicated than I thought...but I think I got it.
The spammers' posts
and if I'm uncomfortable with the rules: Chris' posts
You have it!
Of course, if a noun is already plural, it's different: children's toys, for example; and its is possessive without an apostrophe, in contrast to its non-possessive but abbreviated form (it's is short for "it is").
Isn't English fun?