Everyone's right.
You've got two options. If you don't lie, and declare on your form that you used to suffer from hayfever, then the PMB will consider it on an individual basis. He'll have a good look at all your details when your medical forms arrive. You may find you leave OASC "TMU" for aircrew branches (temporarily medically unfit) pending examination of your records.
If he decides that you've been symptom free for long enough, and the detail recorded by your doc at the time leads him to believe it was mild enough anyway, then it may not be a problem.
If you've suffered recently, then you're probably out of luck.
If you choose option 2 and lie, all that'll happen is your medical records, when examined, will be peppered with references to hayfever. The PMB will realise you've been lying, and rather than having your individual problem reviewed on merit, you'll have your application binned for having a lack of honesty and integrity.
The only potential problem you face is if you've been prescribed anti-hayfever medication recently. It makes it trickier to prove you don't need and haven't suffered if you've been to doctor and said "I have hayfever, give me piriton" or whatever.
Your best course by far is to be completely open an honest. Use the "other comments" box on your medical form to detail when you were prescribed recently, why, all the details you can. In your debrief with the PMB, talk to him at length about it; mention how long it's been since you suffered, and be utterly honest. Talk to him, as you have here, about how you've taken prescriptions as a precaution but honestly haven't suffered. If the doc looked at you at the time, then your records will reveal whether or not you genuinely have a problem.
Every time I reply to a medical thread, I try to make these points:
1. Officers are meant to demonstrate character, integrity, and honesty. Anything involving a lack of these in your application is likely to doom it before it starts.
2. All of the medical conditions which bar you from some form of service or another do so for a very, very good reason. Rushed out for a Tucano flight when suffering from hayfever, and haven't had time to pop some pills? = burst eardrums/knackered sinuses. First solo in high wind when "dulled" by medication? Doesn't bear thinking about!
Also, who on earth was your careers advisor? Find out who his boss is and drop him a line saying you were advised to lie on an application for a commission. I think he misunderstands a little about the selection procedure, which surprises me!