Lower it? Personally I thought it made it higher. The more you talk to them, the higher the level of friendship becomes? Hence why it takes longer and longer for them to need to concentrate on something, because you are more friendly and they will talk to you more before getting fed up xD I don't know though, that's only a theory
If you completely ignore them for about a week after introducing yourself to them, they'll move out on their own. NOT completely sure if they'll prompt you about it or just disappear though!