I had a Norway Maple cut down last year and tried to grow
several plants over and around the stump. All the plants
were eventually engulfed by some fungus and died. I tried
to treat the soil with Neem oil and did not work. I was told by a
nursery that I need to apply a dry fungus treatment pellet that
breakdown periodically in the soil.

Can anyone, Help.

I am thinking of trying to plant Iris next

I wonder if the fungus is just from the stump decomposing? If so, that's what you would want....right? I would either plant a little fuurther out from the stump, or put pots of plants around it until it has decomposed.

Generally the fungus that is growing on stumps is the decomposing kind and actually good for the soil and the plants. The exception is too much of a concentration. Roughing up the decaying stump so the fungus is not so concentrated will help.

I have some fugus growing at the base of one of my tree's. I lost one to the same fungus just a few months ago. Do i use the sick tree treatment?

Ordinary corn meal is organic fungus treatment that works.