You will not be penalized for not including titles and descriptions, but Google will auto-fill them for you.
Takeshi's advice is sound if all you want is to not have these pages displayed in site links (the links under your domain when you google your domain's name). However, it sounds like what you are asking is to not have the pages displayed at all in SERPs ever. I don't quite understand the reasoning behind this, I must say. For example, if you work for Comcast and somebody googles "Contact Comcast" you should want them to land on your contact page. Same with the registration page, I would think..
That said, if you truly do not want those pages indexed you can disallow them in your robots.txt or add this tag to the header: