I think we just need to talk about this often and dig into these problems every day as a part of the job and not let tensions build up.
Address the problems while they are small, use humor and friendly attitude, keep it constructive and everything should be just fine.
I totally agree with you and also think a good approach is to start with asking the offender if he can help with explaining his code to me so I can understand it well enough so I can change it. Maybe also during the talk ask how he managed to test his code. If I am lucky this will make him understand why it is good to have solid code that others can understand and change without his help. This way I am not directly saying that his code is bad, I am saying I am not able to understand it leaving a possibility that it is me there is a problem with. If the code was written more than 2 days ago he will probably also start having problems with understanding exactly what it do.