If you decide not to use the services of this or that site anymore, it is better to delete your account on it. You have to do the same when registering under a new nickname so that the old account does not spoil the resource statistics with its inactivity.
Log in to the site using your username and password. Find an item in the menu called "Options", "Settings", "My Profile" or similar. Sometimes the settings page loads when you click on your own nickname in the upper right corner of the page.
After loading the settings page, select a link on it, which may be called "Account Settings", "Security Settings", etc. On the loaded page, you will find fields for entering the current and new passwords, date of birth and other data. If there is a box for the "Delete account" checkbox, check it, enter the current password in the field provided for it, and then click the "Save", "Update" or similar button.
If there is no checkmark in the security settings page for deleting a profile, look for a separate item in the settings menu for this. By clicking on the corresponding link, wait for the profile deletion page to load and click on the "Delete" button. If after that the question "Do you really want to delete your account" is additionally asked, click the "Yes" button.
On some sites, deleting an account by the owner himself is not provided. This can only be done by an administrator. In no case do not contact him publicly for this (for example, on the forum) - this may be perceived by other users of the site as trolling. Use the function of exchange of personal messages (private) or e-mail. Also, in no case ask to delete all your old public messages along with your profile. Perhaps they provide useful information for other participants.
Finally, if none of the above methods helped you, just do not log into your account for a long time. On some sites, profiles that have not been active for a long time are deleted - manually by the administrator or automatically by the server itself. The period of inactivity of a user, after which his account is deleted, is usually six months.