When you say l2p you're probably just suggesting that they need more practice. Noob basically means the same thing. But that's not the way these words are received.
Whether we like it or not, the internet breeds its own vocabulary, community, and sense of ethics. And words/phrases like l2p and noob are taken as insults. Why? Because they're used as insults. Look all over the web, I guarantee you that these words are not being used as a contraction for a helpful phrase on message boards or in other games.
I'm pretty sure that f*** started out as just a word too.
Also consider this. When online we have a tendency to overreact to things because we, as people, have lost our ability to feel empathy. This is caused by our inability to see the other person's facial expressions. (Yes there are studies on this, no I won't go find them for you I'm tired.)
Further, consider the kinds of people you want to play this game with. Do you want to play with level-headed people, or the hot-tempered.
So instead of just blurting out a phrase that you KNOW has become an insult. (Don't pretend you don't know.) Try being the calm one, express a full thought. If they turn into the hot-tempered one after that then don't intentionally egg them on, don't pursue their anger with your own. Remain calm, be witty, or just /ignore. And if you honestly didn't know it was insulting then just apologize and move on.
1. Face the facts. Certain words have become insults due to their use.
2. People online have a tendency to overreact, recognize this and watch yourself.
3. Try to remain the level-headed one in arguments.
4. Don't pursue their anger with your own, /ignore if necessary. At least, it takes a lot less time.