A harmonic is not noise. And an isolation amp is not going to suppress a harmonic unless it's bandwidth is way less the the harmonic. I think what you want is a filter.
Can you explain what you are doing and what you want to achieve? The question is so short and vague and as such difficult for us to offer reasonable help.
Like Barry has said, you need a filter at the input of the converter. If it's about the control loop, you need an integrator to implement an inverted-zero at a frequency higher than the harmonic that you want to attenuate.
The question was posted in the Power Electronics Forum so I suppose he is talking about a 50-Hz line frequency and its 150-Hz harmonic. Other than that, then there's a likelihood that the question is posted in a wrong forum.