This can happen when...
You have a fairly generic email address, e.g., bruce@wayne.com
Someone posted a guest comment using a fake email address without realizing the email address they used (yours) is real.
Another person has a similar email address to yours and misspelled theirs when posting a guest comment or creating an account; or
Someone posted a guest comment using a fake email address without realizing the email address they used (yours) is real.
What do I do if someone made a full Disqus account with my email address?
Enter the email address into the box at our password recovery page. You should get an email within minutes at the address you entered into the box. If you don't, check your spam filter and make sure you're not filtering emails from Disqus in any way.
Using the link in the email, choose a new password. This prevents the person from using your email to continue to make posts.
Delete any comments which were made with the account before you took control of it. Keep in mind that this will remove identifiable information from the comment (no name, email, or avatar) but will not remove the comment from the discussion thread. Visit Removing and Editing Your Comments for more information.
Note: We recommend not deleting the account. You're not required to use the account but keeping it registered will prevent others from re-registering an account with your email address (whether intentional or not).
What can/should I do with these comments?
Once you've taken control of the account, you can delete each comment manually, which will prevent it from appearing on the page at all, or you can delete the account, which will anonymize the comments, stripping them of all user information. See Removing and Editing your Comments.
How is someone else able to post with my account?
Guest users can post under any name, as display names do not have to be unique. More information on the difference between usernames and display names can be found here.
How can I block others from commenting using my email address?
It's not currently possible to block guest comments posted using your email address. However, Guest comments are separate from accounts registered with the same email, so these comments shouldn't affect your Disqus account.
If your email address is being used for a Disqus account that you did not register, we recommend taking control of the account through a password reset email and leaving an account in your control so that the email address cannot be re-registered.