How do you get Steve to stop following you in Hetaoni

You can't - Steve is designed to appear at random times and to chase you around. You can avoid running into him by running in the opposite direction (shift + arrow key). This is fairly easy to do as the game is designed so that your characer's speed can outrun Steve's, given that there's still some space between you two. The creepy music will also start playing when Steve is going to appear, so take it as a warning and get moving!

To get Steve to stop following you, you can run to the bathroom in the first floor and hide there. Steve will disappear when you go back outside. You can also run around the mansion and you will lose him soon enough.

As usual, remember to save regularly! It also helps to look at walkthroughs on Youtube to guide you.