The Rails generators just create the necessary text files, with the script entries required to do the basic job. There is nothing to stop you modifying this after they have been created. In fact, you are most likely to do so, as the standard set will get you started, but it unlikely to do all you want unless you have very basic requirements.
However, you can also undo a generate by using the 'destroy' option rather than the 'generate'. So
Have a look at this:
# Create a model call Ball
ruby script/generate model Ball
# Undo the generate
ruby script/destroy model Ball