This is an easy one. All you need to do is remove the information in the file of the person who sent it to you. Here are the 5 steps from Gizmodo:
1. Make sure that your Zune is connected and the Zune Software is running.
2. Navigate to My Computer and click on your Zune's name.
3. Open "Storage"
4. Find the file that contains the name of the user who transferred songs to you in the file name, and delete all records with that title.
5. Get the other party to do the same and you can now transfer the same songs again!