From AVR Studio Menu choose VAssistX -> Options -> Advanced -> Underline
Uncheck the highlighted entry
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.