I just got Pierce from another town... but he moved in on my stump set-up right next to my house... I've heard there are ways to change this but how do you do that?
If you've already loaded up the game with your mayor, his house is stuck there and can't be moved. Sorry :/
Here's a guide to what you're talking about: boop. Read the second part, called "Determine a new villager’s home placement" and it explains it pretty thoroughly.
There isn't a way to change it. There's ways of preventing it, but once it's already happened, there's no way of changing it.
For the future reference, on a day someone is going to be setting ropes up for their house, start a new character. Walk around the town until the find the place the ropes are. If you like the spot, save, then play on your main character. If you don't like the spot, reset and start the game with a new character again. Keep doing this until it's in a place you approve of.
Once you load the game on a character already living in your town, the town is loaded and saved for the day and even if you turn off without saving, there's no going backwards from that point.
You can't change it after he's plotted or moved in if you loaded the game as your mayor. The only way to do this now would be by moving him out, cycling through 16 villagers, and getting him back again. When you load your town as your mayor or as another existing character, the state of your town is saved, which is why you can't reset for a different plot location after you've done that. However, if you use a new character, the state of your town won't be saved until you save, which means that you're allowed to reset for different plot locations or villagers.
There are plot resetting guides so you can prevent this from happening in the future, like the one toadsworthy linked to. You can do something similar if there's a chance that someone new might move in uninvited, if you're afraid that they're someone you don't want or that they might plot their houses on a bad spot.
