If you have Site Settings permission level, you can manage all Comments posted in Discussions and Forums via the Comments Admin area. To access it click on the Site Menu and choose Comments.

You will see a list of all Comments made across the site. To open the page containing the comment, click on the relevant row in the table.

Comments can be filtered and sorted. To remove a comment, click on the trash bin icon. Once removed the comment will be greyed out and struck through. It will no longer show on your site.