First, determine which version(s) of Cocoapods you have installed by running this in Terminal:

gem list --local | grep cocoapods

You see output similar to this:

cocoapods (0.27.1, 0.20.2)
cocoapods-core (0.27.1, 0.20.2)
cocoapods-downloader (0.2.0, 0.1.2)

Here, I have two versions of Cocoapods installed.

To completely remove, issue the following commands:

gem uninstall cocoapods
gem uninstall cocoapods-core
gem uninstall cocoapods-downloader

If you have multiple versions installed, like I have, it will prompt you to choice a specific version or all. If you want to uninstall a specific version you can also use the -v switch as follows:

gem uninstall cocoapods -v 0.20.2

Running gem list --local | grep cocoapods again will confirm that Cocoapods has been removed.