See Surround Selection for an applicable setting in the options dialog of Visual Assist.
Visual Assist build 2059 and older
By default, when you use the Surround Selection feature to comment out several lines of code with // comments, Visual Assist adds // and a space to each line. This extra space is automatically removed when Visual Assist uncomments the lines. You can tell Visual Assist not to add the space by changing the registry key:
HKCU\Software\Whole Tomato\Visual Assist X\<IDE spec>\DontInsertSpaceAfterComment = 01
Set this to 00 for the default behavior.