Unfortunately there is no way you can undo that, and only you can do manually, but i have some suggestions you can try which i did
You can search for that package so for instance, i cloned one rendering "Free Hand HTML" and also cloned one component called "Plain HTML" and created my own module called "Free Hand"
Now if after doing all these, basically it creates templates, renderings, branch templates, Media library folders etc, so there are multiple reference to it
You can simply search for your module in sitecore search, you will find all places in the content tree, go and remove them manually
Thanks,
D