I suppose that you are talking about the client side, new ticket page.
As soon as the client chooses the help topic, a new form is loaded and you want to hide certain fields on there.
You are going to need JQuery for that and the only way I know is with Motherload.
We are already doing something similar with our Superlists plugin.
Basically, what you have to do is check whether the client satisfies your criteria and if so inject a JS function into the page to hide the fields.