SSMS shortcut to comment/uncomment a specific part of the query?
You can easily do this with Code Snippets, something like this will work for your example:

&lt;?xml version="1.0" encoding="utf-8" ?&gt;
&lt;CodeSnippets  xmlns="http://schemas.microsoft.com/VisualStudio/2005/CodeSnippet"&gt;
    &lt;_locDefinition xmlns="urn:locstudio"&gt;
        &lt;_locDefault _loc="locNone" /&gt;
        &lt;_locTag _loc="locData"&gt;Title&lt;/_locTag&gt;
        &lt;_locTag _loc="locData"&gt;Description&lt;/_locTag&gt;
        &lt;_locTag _loc="locData"&gt;Author&lt;/_locTag&gt;
        &lt;_locTag _loc="locData"&gt;ToolTip&lt;/_locTag&gt;
        &lt;_locTag _loc="locData"&gt;Default&lt;/_locTag&gt;
    &lt;/_locDefinition&gt;
    &lt;CodeSnippet Format="1.0.0"&gt;
        &lt;Header&gt;
            &lt;Title&gt;SLASH STAR COMMENT (surrounds)&lt;/Title&gt;
            &lt;Shortcut&gt;&lt;/Shortcut&gt;
            &lt;Description&gt;Surround selected text with slash star comment block /**/&lt;/Description&gt;
            &lt;Author&gt;wBob&lt;/Author&gt;
            &lt;SnippetTypes&gt;
                &lt;SnippetType&gt;SurroundsWith&lt;/SnippetType&gt;
            &lt;/SnippetTypes&gt;
        &lt;/Header&gt;
        &lt;Snippet&gt;
            &lt;Declarations&gt;
                &lt;Literal&gt;
                    &lt;!--&lt;ID&gt;x&lt;/ID&gt;&lt;ToolTip&gt;y&lt;/ToolTip&gt;&lt;Default&gt;z&lt;/Default&gt;--&gt;
                &lt;/Literal&gt;
            &lt;/Declarations&gt;
            &lt;Code Language="SQL"&gt;
                &lt;![CDATA[/* $selected$ $end$ */]]&gt;
            &lt;/Code&gt;
        &lt;/Snippet&gt;
    &lt;/CodeSnippet&gt;
&lt;/CodeSnippets&gt;


Save the file with a .snippet extension and follow the instructions from here to add the snippet to SQL Server Management Studio (SSMS).  

Instructions for use


highlight the text you want to surround with the comment block in the query editor
Press Ctrl+K, Ctrl+S to summon the 'Surround With' snippets.  Note the comma between the two sequences, it means "press Ctrl+K, then press Ctrl+S in quick succession"
Select your snippet from the list
Your highlighted text is surrounded with the comment block
press Ctrl+Z to Undo - this is the normal windows undo command.  It will remove the comment block as long as you haven't made any other edits.