In Dragon Ball Z, collecting all 7 dragon balls allows you to summon an all powerful dragon that will grant you one wish, for anything you desire. Wishing for someone's death or resurrection are not limitations
The dragon (at least Earth's dragon, Shenron) has been asked for feats that were too great for him to perform, though, and had to refuse the wish stating they were "beyond his power."
We don't know the limits of his power re: ass kicking. But it's probably unlikely that Shenron could delete Frieza. Possibly Porunga (Namek's dragon) could have. Who knows.
No he couldn't kill King Piccolo because he cant kill those that are more powerful then his creator. Location doesn't matter. And i think Goku once refused to be wished back to earth because he was training somewhere in space, meaning shenron couldn't overpower him
I'm gonna have to do some rewatching on the king piccolo thing, but I always took that as Shenron just respecting goku's wishes, because they have known each other for a long time.