From AVR Studio Menu choose VAssistX -> Options -> Advanced -> Underline

Uncheck the highlighted entry

How to disable spell check in comments in avr studio 5.0

If you have only specific words that you want to add to existing dictionary (or) add a new dictionary on your own please follow this post. http://www.wholetomato.com/products/features/misspelled.asp.

Downloading dictionaries to atmel studio and installing them (Source : WholeTomato)
Download your additional dictionary from OpenOffice.org, unzip and copy the dic and optional aff files to theDict subdirectory of the Visual Assist X data directory:

Vista/Windows 7:  C:\Users<code>%USERNAME%\AppData\Roaming\VisualAssist\Dict
Windows 2000/XP:  C:\Documents and Settings<code>%USERNAME%\Application Data\VisualAssist\Dict\

Restart your IDE to make Visual Assist X find your new dictionaries.