Rather than trying to get Ant to remove some classes from your class path, why not attack it from the code - side, which is better practice anyway (you no longer would be dependent of the build system in order to get correct code).
Your problem is that you need to use xxx.Currency instead of java.util.Currency. There are two solutions:
1) Don't import java.util.Currency into your source files. This would include NOT doing this:
Instead import the individual classes from the java.util package you need:
2) Use a fully qualified class name whenever referring to Currency. The fully qualified class name is the name of the package + dot + the name of the class. So java.util.Currency or my.app.money.Currency. Example:
I always try to do the first option (don't import 2 classes with the same name) whenever possible, but there are times when I need to fall back to the fully qualified names as well.