Can an unwanted post in a Discussion be deleted?

Yes, KB comments can be deleted by:

 - A user who has the 'Create article' permission in a KB category; can delete someone else’s comment on articles they’ve created in that category.

 - A user who has the 'Delete all articles' permission in a KB category; can delete someone else’s comment on any article published within that category.

A user that fits the above scenario(s) can delete a single comment straight away using the 'delete' function:

However, if there have been replies to a comment these will need to be removed first before the ability to do so for the original comment will appear under it.

How do I pin something to the top of a Discussion?

It is not currently possible to pin a comment in a discussion, but a workaround is to edit the description of a Discussion topic and put in the link/information you wish to be 'pinned' as it will always show at the top: