From the FAQ on Meta.SE:
Who can delete comments?
A user may delete one of their own comments at any time by clicking on the (x) icon that appears to the right of the comment's timestamp when the comment is moused over.
Comments that are flagged by multiple users are deleted automatically. The number of flags needed is based on the comment's score. It currently takes "
3 + (Score / 3)" flags to delete a comment. Comments containing certain keywords can reduce this amount.
Moderators can delete any comment, or purge all comments from a post. They also have the ability to move all of a post's comments to chat.
When should comments be deleted?
Comments are temporary "Post-It" notes left on a question or answer. You should not expect them to be around forever: Once a clarification has been made, an edit added to the post to include new information, or the issue in the comment is otherwise resolved, it is subject to deletion. In reality, many obsolete or chatty comments remain untouched due to the high volume of comments posted, but this does not mean that they can't or shouldn't be deleted in the future.
So if you see an outdated comment, flag it as "no longer needed".
no longer needed
This comment is obsolete, chatty, or otherwise unnecessary.
The system (either the moderators or your fellow users) will handle the rest.
Flagging a comment:
- On desktop, this is done by clicking on the flag symbol that appears when you move your mouse cursor to the left of the comment (currently not possible on the mobile version of the site), select "no longer needed" and click on the "flag comment" button.
- On mobile, this is not possible, except by going to the desktop version or by using the app.
- In the app, flagging is done by tapping to select the comment, then selecting the option to flag from the menu.