[Solved]-Disable marker hover in only one marker of highchart-Highcharts

Try this in your series: enableMouseTracking: false

In your case, it would be:

series: [{ 
    data: [15.0, 6.9, 9.5, 14.5, 18.2, 21.5, 25.2, {
        enableMouseTracking: false,
        y: 26.5
    }, 23.3, 18.3, 13.9, 9.6]    
}]

I hope this helps!

(Update on 6/23/17): For those who want to apply this to every series in the chart, not just one, you would do the following:

plotOptions: {
    series: { enableMouseTracking: false }
}