You can delete objects in the graph by issuing HTTP DELETE requests to the object URLs, i.e,
DELETE https://graph.facebook.com/ID?access_token=... HTTP/1.1
To support clients that do not support all HTTP methods (like JavaScript clients), you can alternatively issue a POST request to an object URL with the additional argument method=delete to override the HTTP method. For example, you can delete a comment by issuing a POST request to https://graph.facebook.com/COMMENT_ID?method=delete.
You can delete a like by issuing a DELETE request to /OBJECT_ID/likes (since likes don't have an ID).