How do I remove a package installed via Chocolatey?

Use

cuninst packagename

See Uninstall Command for details.

UPDATE: as per @ferventcoder's comment, Chocolately has added support for uninstalling packages since this question was first asked and answered.

The syntax is chocolatey uninstall packageName or just cuninst packageName.

You can optionally uninstall a specific version:

cuninst packageName -version 1.0.1234

See Rob's link for the latest, including known limitations: https://chocolatey.org/docs/commands-uninstall