What is a polite way of telling guests they can't just invite extra people?

We're having a problem with a lot of our guests. Unfortunately, me and FH are young (22-23) and a lot of our friends haven't been to a wedding before and don't know proper etiquette. We have had a couple friends (that we weren't that close with and weren't planning on inviting) post their address to our facebook wall saying "Here's my address! I know you're sending out STDs! So excited to get mine!" Etc. Now, someone who we haven't heard from in over a year (not a peep when me and FH got engaged OR had a baby) is upset that he didn't get an invite and asked someone else that was invited to the wedding if he could be their plus 1. Ugh! I really don't feel like hurting peoples feelings... but I don't want to keep paying for extra people. We got another message today that a groomsman wants to bring a date to the wedding but he doesn't know who yet (cause he isn't with anyone). Is it bad that we're only allowing couples that have been together for at least a couple months? Any advice?