You can remove your Nextcloud account by logging in to your account via web interface then in the top right corner in drop-down menu select "settings" then in left menu click on "Delete account".
To remove account fallow this steps:
Login to your Nextcloud Account
In the top right corner click on your avatar and username and the drop-down menu will open click on "Settings".
A setting page for your profile will open. In the left menu select "Delete account".
After that you will have to confirm the deletion request as shown bellow. After clicking on "delete my account" your account and data will be permanently removed and you will be logged out.