Presumably you didn't set up spam protection measures. ZergSpammers automatically scan the web and register accounts whereever they can.
There are a number of spam protection plugins for Q2A. Some are listed in the documentation, and there are also SPAM Registration Stopper by pupi1985 and Registration Blocker by Yours Truly.
I recommend you install and configure either pupi's or my plugin, require account confirmation, and also set up a CAPTCHA. The most commonly used one is Google's reCAPTCHA, but other (lesser known) ones should work too (perhaps even better, since spammers tend to develop automated workarounds for widely used anti-spam measures).
But even with those in place, you'll still get some spam account registrations, so be prepared for having to do some manual cleanup every now and then.
Since on second glance your question is a bit ambiguous: if you're asking how to remove the accounts that already have been registered rather than how to protect your site from more spurious registrations, there are basically two approaches.
- You can write a custom PHP script to find and delete them (see here for an example of how to remove unconfirmed accounts older than x hours).
- You can delete them in the Web UI. You'll want to install the User Manager plugin for that, so that you won't have to block and delete each account individually.