I just got the same exact crap treatment a few weeks back. It was a terrible feeling because it was not just one wannabe pro photographer but three! Best part was one of the so called leader of the pro was giving lessons to the other two during my assignment! I completely understand how you must have felt that day.
At times, I wished there was some form of law or charter to protect us from jokers like these. But upon further thinking, it (a charter) would not solve the problem but create more because there will be more jokers who pops out every other minute thinking they got some of protection. Alas, subject to abuse which leads to more confusion and frustration. No thanks. There was a thread about this a long time back I think.
Anyway, lesson learned was the bride was the one that needs to be educated. Telling off these wannabes pros is not going to work because they think that they are doing a big favour to the bride by showing up. I am guessing the bride just said yes to everything their relatives', friends' etc request so until you pull the plug from the sink, crap is filling up every minute. Some bride even think having more back up photographer is good like in my case, it went the total opposite. Another bitter assignment I had to cover a couple of years back was the bride told me to give way to this slightly more professional guy just because he was her uncle. In the end both the bride and uncle are jokers because they spent too much time doing posing shots upstairs and guests were waiting downstairs who pretty much left without saying goodbye. Some pro he was.
On the flipside, since I am already paid and these jokers just pop out of nowhere thinking they can do the assignment better than me for free, I would just let them. Saves me a lot of trouble getting the good shot and I can do my other work. LOL! Some crazy thought... just kidding la.
What you should do in an event like this:
1. Explain to the bride the situation because keeping mum only escalates the problem.
2. Stay calm and continue to do your work to the fullest. Last thing you want to hear is an angry customer.
3. If wannabe pro gets in your way, do it like I do. Get in front of his camera and get your shot.
4. If wannabe pro is persistent, you react the same way too.
At the end of the day, this bitter assignment is only one out of the many great assignments you will get to do. Take it like a man and I am sure you will drift through it. All the these bad experiences are just like challenges in your everyday life but don't let them eat you up. Hope that helps and welcome to the club. LOL!