You can do this pretty easy with git itself.
git clean -df
You only need to pass
-d if you have untracked directories. You only need to pass
-f (force) if you have git configured with clean.requireForce set to true, which you likely do.
To avoid removing files unintentionally, run the following first
git clean -n
This is a 'dry run' and it tells you which files git plans on removing before it actually removes them.
If you want to be picky about how you clean up your untracked files you can use
git clean -i
This will give you options, including removing files that match a pattern.