Alexander... how would you define the default import packages?
Arek, if you use a subclass of GroovyClassLoader you can do it like Alexander did.
As for addImportPackage(), this wants to have the packe plus ".". That is because of historic reasons, so don't ask Anyway, if you want to import foo.bar.*, then you have to call addImportPackage("foo.bar.")
As for preventing a user to use certain classes. I suggest you add a phase operation that checks each ClassExpression for a usage of that class. In the Groovy ditstribution since 1.6-beta2 and 1.5.7 you should find in the examples directory an example called ArithmeticShell. This shell does for example forbid the usage of certain classes. So look at that example.