According to my RHS book you can't, although other places say treating the soil with a fungicide prior to planting might work. It is a fungus which causes the stem rot (commonly called pansy sickness) and it builds up in the soil - if your pansies are planted in containers, then the compost can be changed, the pot washed and sterilised, and fresh compost used each year. RHS recommendation for planting in the ground is to plant pansies in a different spot instead.

Thanks, much appreciated.