Even if you block everything, they will find their way around it because having a social presence is very important for teenagers, even if you don't consider the content valuable. It's part of belonging and growing with their peers. Limiting access is probably a punishment ("tough love" if you want) but not a lesson. You can help them realize Instagram is full of crap and let them chose to avoid it. Kids are smarter than we think, and trusting them is more powerful than building walls (although it's true that it's more difficult and scary just trusting them).