how to fix this

Can you use a heat gun w/o melting the thing? IIRC methylene chloride will loosen Loctite but it's not the easiest chem to source and you'd need to be sure it wouldn't harm the plastic knob itself, unless the knob can be replaced by another knob altogether (which might not be a bad idea). There are all sorts/types of knobs available that are held on w/set screws, etc.