Easy, stop playing 3v3, lol.
In 3v3s and 4v4s your chance to encounter trolls and other non-serious guys is pretty much 100%. People who play the game "seriously" and want to get better play 1v1, sometimes 2v2. People play 3v3, 4v4 and FFA just for shits and giggles.
Well, I play 3v3 because it seems to be what Blizzard want new players to try first. Campaign, challenge, co-op, 1v1. I think the wikia page said so too, saying it's the way they want to introduce new players to the game.
I'll probably stick to the campaigns and challenges for now... until they finally allow SEA users to get NA client as well. Then I will play my friends over there.
Also, how did they manage to get killed by the easy AI? Does that thing actually attack?
They wasted time trolling... and a monkey at the keyboard will probably play better than they did. They didn't scout at all - it was my orbital command revealing the AIs to them. Only one of them tried to harass with a small ground of zerglings... from the looks I think they weren't speedlings. And AIs (at least one Terran) responded quickly with a bunch of hellions, wiping one of them out. I led Terran AI back to the remaining troll, and the Zerg AI joined in. The troll still had nothing, AI Zerg had roaches and hydralisks.
When both trolls were gone, I moved my base to another area. Zerg AI quickly followed with a bunch of brood lords.