Data Validation: Validate against INVALID words instead of valid words

I have a data validation question that I need some help with. Basically I am wanting to create a list of words that you CAN'T enter into a cell, rather than a list that you can. For example, I have a code that I want Excel to prevent being entered, and the code is DUN502. I have attempted to go to the custom section in data validation, and use this formula:

Please Login or Register  to view this content.

Which does not work.

I want this to work in the case where I may have other items to add on to the list, so something that would allow an and statement such as:

Please Login or Register  to view this content.