? buyout everything in the flea market at retail and fill all the spaces with flowers priced at 999,999 bells.
this will make villagers unable to sell their furniture or to buy someone else's.
? change all of the custom designs in the able sisters to umbrellas. this will keep villagers from changing into those
awful default designs that we all love so much.
if it's a cycling town, don't talk to the villagers or do favors for them either. if it's a regular town, you can just deny doing favors, catching bugs/fish for them as well. but obviously you'll want to talk to them in a regular town c: just keep in mind there's always a chance villagers can swap catchphrases and shirts, especially in an active town you're playing normally in. it's almost inevitable that that'll happen eventually.
Don't let the villagers buy furniture and dinosaur bones from your pocket. I have made that mistake several times only to see them put the items in their homes. Best thing to do is not have the items in your pocket for long because eventually someone will ask you if they can buy them. I run to the Re-tail or avoid villagers so they don't ask, some of the villagers will get depressed when you deny their request.
Of course you can do all the right things so they don't change their home but somehow there are those villagers that will change things anyway. Julian changes things in his home even though I've never given or sold him anything. My retailers full of expensive art so he did't get it there, he's magical.
Just literally don't let them buy or sell you anything.
Even then unfortunately it's not guaranteed as they will often get gifts from other villagers.
Possibly by not delivering parcels for them when they ask you may help avoid that though