dude, blocking people is never a good idea. It can lead you to more problems. Like, a friend might tell them ur online and she might be like "wtf, he's trying to avoid me?" Or maybe she has another sn you don't know and she can see your actually on but you blocked her.
Just use away messages. I'm on like 24/7, but 22 out of the 24 hrs, or even 23 sometimes, my away message is up. But don't tell people your life through your away messages. Like "going to school 7-3, then practice, then food, then back." The less you say, the more people will wonder what you're up to, and think about you. Like, the only away mesage I ever use is "gone. bye"