This is how you delete comments:

If you come across a hate-posting that in your opinion contravenes the guidelines set by Facebook, you can report it. And you should do too. Because that’s the only way you’ll get the message to disappear and stop offending, hurting or intimidating people. Under every single comment, video or picture on Facebook, there’s a button with which you can report the contents.

Then, all you need to do is to give a simple reason for your reporting of the comment: is the comment directed against religious groupings, racial groups or people with a particular sexual orientation? Your report will be checked by Facebook. Unfortunately not ever hateful comment contravenes Facebook’s guidelines. But it’s always worth a try.

Here you can find more information:

Report to Facebook