This isn't something particular to just MVC - it can affect any part of your system where a junior developer isn't following your team's standards and guidelines. The way to catch issues like what you describe is to have procedures in place for junior developers to learn and expand their craft while not hurting your project at the same time. This includes, as Dan mentioned in the above comment, regular code reviews of their work, but can also include ongoing learning sessions for your entire team where, say, someone can present work their recently did on a new technology that shows how or how not to use it, etc.
So that the junior developers don't feel that they are being unfairly scrutinized, the code reviews should be presented to them in a way that shows how they are for everyone's benefit:
- The junior developer will learn coding practices they may not have been aware of.
- The junior developer may bring new, clever ideas to the team that no one thought of before, as people often develop coding habits over time that tend to make them close-minded to new ways of thinking.
- The team in general benefits from better communication, knowledge sharing, etc. While some may view code reviews as an impediment to getting work done, in the long run, they should save more time than they take up.