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.
Addendum:
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.
