Yes, you can block specific users from commenting on presentations by following the steps below:

• Navigate to the presentation
• Scroll down to the comment
• Mouse over to the comment made by the user you want to block
• Click on the block link

The blocked user will no longer be able to comment on your content ,send you private messages, post on your wall, send you invites to groups and events, share content with you, follow you (or will get removed if already following).