I don't think the auto-fill can be turned off.
In the help (Master Location List) it says:
"When you enter a location in a Place field, Legacy checks to see if the name is already in the Master Location List. If it is not found, it is added to the list."
You do need to take care when entering names, but it does get easier with practice. Some places you have to type in more to get to the one you want, than others.
Sorry I can't be of more help at this stage.