Resolved inline comments cannot be deleted. You have to reopen (unresolve) the comment and then delete. There should be a "Delete" link in the Resolved Comments window so you can delete without having to reopen first.

To reproduce:

• add an inline comment to a page
• resolve the comment
• view the resolved comment via the page more actions menu - there is no way to delete it but there should be
• reopen the resolved comment
• now you can delete it via the inline comment more actions menu

Workaround

To delete inline comments that were automatically resolved when the highlighted text was deleted on the page, you can issue a DELETE using the REST API. For example:

curl -u adminuser:password -X DELETE "BASEURL/rest/inlinecomments/1.0/comments/CONTENTID" | python2 -mjson.tool

You'll need admin credentials. Replace BASEURL and CONTENTID with the appropriate values. You can find the content ID of the inline comment through a browser network console or by issuing a database command. If you know the ID of a page, you can find its inline comments and their IDs with the following query:

select a.contentid as "comment ID", b.body as "comment body" 
from content a
inner join bodycontent b on a.contentid = b.contentid
inner join content c on a.contentid = c.pageid and c.contenttype = 'CUSTOM'
where a.pageid = 123456 and a.contenttype = 'COMMENT';

Written for Postgres. You may have to alter it slightly.