Hi Steve,
in this scenario, we would probably want to use a JavaScript extension scoped to the SiteCatalyst tag.
I'm going to make the assumption that your "client side code" contains a variable with a unique value that would be passed to SiteCatalyst or otherwise 'notify' that the SiteCatalyst would fire.
In a JavaScript extension (don't forget to scope it to the SiteCatalyst tag!)
use the following script:{code:javascript}
// logic to check if my variable does not equal some value
if(b.myvar!="somevalue"){
return false;
}
{code}
where b.myvar is a unique variable in your data layer that when populated with a specific value means that it's 'ok' for the SiteCatalyst tag to fire. (b is the short-hand notation for the extension's locally-scoped copy of utag.data)
the JavaScript "return false" will prevent the tag from firing.
FYI, this is the simplest execution of this method. You may have to write additional logic if the specific variable/value you are evaluating in the "if" statement is not available or populated at the time that the extension executes.