First of all, have a good reason for thinking something is being done "too much". It's easy to over-react. When I was a kid, my folks took a lot of guff from other adults for letting me "play on the computer" so much. I started programming when I was six and now make a career of it. Think about:
What effect is the activity having on your child? Is it causing him/her to miss out on more important things, or impairing his/her ability to do well in school?
Is the activity productive in some way? There's a huge difference between playing games and learning to code, between goofing off reading webcomics and surfing for ways to make your own drawing better, and so on. Productive activity is by nature more valuable than unproductive activity.
Why is the activity so important to your child? Escapism is concerning. Getting a sense of accomplishment from "fake" activity like an MMORPG is concerning. Developing a skill (even one without obvious practical value) and real achievements are good. (Of course there are many other good reasons to do things, I'm talking specifically about good reasons to do something in what others my see as excess.)
If something really is a problem, don't force your teen to quit (that will likely backfire). Instead, supplant it with another activity that your teen can get excited about and will get more from. You may have to get him/her to try many different ones until something sticks.