Some PC barge into a house and takes whatever he wants, the man inside could try to stop him (but if the PC is well equipped, the guy may flee to the street requesting the guards to help him), the PCs can take whatever they want in the house, but they won't find much in the house of a commoner), they leave without hindrance.
Then you may put a lot in motion.
- the guards may stop the PCs in the street asking them to follow them - they look like the described suspects
- a bounty may be put on them, a bounty hunter usually strikes when least expected
- the victim may recognise them later in the street crying "Guard! Guard!"
- he may also recognise them in the tavern, creating a big brawl requiring the local guard to stop it, then... they have to defend themselves against the law
If the PCs hide behind their status, they may be warned that their behaviour may lead to dire consequences, or they may simply be sent on a very dangerous quest, hoping that they will never come back.
If your PCs kill the man right away, you may have other witness like children upstairs, a wife, some teens in the next room, in medieval times, family tends to have several offspring. Children run into streets disappearing rapidly, passing where a full armoured PC can't.
Even if the PCs leave no witnesses, even the beggar facing the house or at the corner in the street, the murder of an entire family is bound to attract attention and investigation.
They may have the means to wipe out the city, but there you can have a NPC teleport out to next city, warning them of their incoming doom. Congratulations, the PCs are persona non gratta in each city of the country.