If you do decide to ban social media, you need a pretty clear strategy over how you're letting guests know. You have probably worked out you're going to be busy in the run up to the ceremony, so you need to nominate someone to break the bad news to any Instagram addicts, and they need to be a) important enough and b) bossy enough to make the rule stick.
You could factor your rules into your stationery and have (friendly) signs made. Or you could choose a confident bridesmaid or groomsman or even ask your registrar or photographer. Just bear in mind they won't be able to police the decision for you, and there are only so many times they can ask before everyone starts to get uncomfortable.