I'm assuming you're using Ubuntu, or at least something Debian-based with APT as your installation instructions indicate. In that case, I'd recommend reading this answer (a bit dated but will still work) to find the best option for you:
(Here, "nzbdrone" is "packagename".)
Casual Googling tells me you may also wish to delete /opt/NzbDrone if it still exists after this, so:
sudo rm -r /opt/NzbDrone
After doing this, you'll still have the repo listed in your sources and their key added. This isn't really a problem; it just makes it available to install if you explicitly do so again in the future (so you could skip that first step). If that bothers you:
Be very mindful when using "rm -r" , watched an "experienced" admin run one from root. I usually end up changing into the parent directory and just use "rm -r foldername".