They were probably automatically created by spam bots. You can disable user registration by going to the Settings > General page and unchecking the Anyone Can Register box. You’ll still be able to manually create users, but users won’t be able to register themselves automatically.
If you want to allow users to register, but still avoid spam bots, you can use a plugin like Pie Register to setup a CAPTCHA on the registration form.
To remove the existing spam users, you can use a plugin like User Spam Remover.
For more information and ideas, see these existing questions:
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