How to disable spell check inside comments in AVR Studio 5.0 ?

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.