I think this should work:
$If($IsMatch(<Custom1>,"x"),,<Custom1>)
And you could add something that might make it more solid for some uses:
Suppose the 'x' from your example is 'jazz'.
And you want to have your virtual tag shown empty only when the custom tag contains only and exactly 'jazz', but you do want to display the contents of the custom tag if it contains 'jazz rock' or 'acid jazz'.
Then you could use:
$If($IsMatch(<Custom1>,"^jazz$"),,<Custom1>)