Well first, don't try to blame all of it on Janna. Yes, some times someone feeds making it seemingly impossible to win, but you can't lost a game by only 1 person feeding. Multiple people on the losing side have to contribute to lose, it doesn't all fall on one person... Now, I'm not saying you did anything wrong, but keep that in mind.
Now, About the Mejai's. Having Soulstealer on Janna is actually really good, if you can actually get use out of it. Having a point/click Slow and point/click Shield, it is very easy to get assists (which get stacks of Mejai's.) Now, if you're losing more stacks than you're gaining, then you have a problem and probably should sell it. In this situation, you should probably get sightstone before anything (Actually, you should rush sightstone before anything) and she probably should not have gotten Mejai's at all.
In reality, wards hardly cost any gold at all. I get 1-4 wards every time I recall and almost always have enough gold for what I need. It is also not **only **the supports job to ward. In fact, every single person needs to contribute to buying wards, so you were in the right there.
Unless you were flaming, you were not in the wrong here. You all bought wards, you all did your part, it's not your fault that Janna fed but you also could have been more cautious and gotten yourself fed instead.