Turn off +/- outline symbol for one line scope

I am trying to define visual studio style multi-line comment state:

<Scope>
<ExplicitPatternGroup Type="StartScope" Token="CommentStartToken" Style="CommentDelimiterStyle" PatternValue="'" />
<RegexPatternGroup Type="EndScope" Token="CommentEndToken" Style="CommentDelimiterStyle" PatternValue="\n" LookAhead="[ \t]*[^!]" />
</Scope>

I also implemented my custom semantic parser to enable the outlining. It works great.

I have a question though. Is there a way to turn off the +/- outline symbol for single line comment since there's nothing to expand and collapse? Showing the +/- for each and every single line comment is distracting.

Thanks.

Steven