Comments on your profile can be deleted by pressing the trash can button on the comment you want to delete. You can only do so for your own profile, but mods and admins can delete comments elsewhere.
You'll have to delete them all individually. That is the only way. Though unless you have comments that you would like to not have on there then you shouldn't have to worry about it. You won't run out of room in your comments section anytime soon.
With patience. I can however recommend opening the deletion in a new tab as that will make the process marginally faster. Simply control+left click on the trash can for each comment (at least in Firefox), and it will delete without you having to go through a lot of unnecessary steps like going back to the all-comments display and so on.
I do however recommend warning your friends about this, so they won't come complaining about losing their points here on the forums, or on mod-walls.
To do this faster, ctrl+ - or ctrl+ scroll up to make the page much smaller. Then all the check marks and comments will appear more on one page. you can then scroll to cover more ground since more checkmarks will appear next to eachother.
After you're done, hit ctrl+0 to return to normal view.
