You can't set "create package" rights so to speak. When you save a package you can set a user password, which allows you to just run the package, and an owner password which allows you to edit the package.
Also, don't install client tools (eg Enterprise Manager) for people that don't need them.
Remember, even if a user creates a package, they can only execute queries that they have permissions for anyway. So if they can't select from a certain table thru Query Analyzer then they won't be able to read from it thru DTS.
--James