I have mushroom taking over my back yard. I need some advise please on how to get rid of them.
Mushrooms are simply the reproduction organs of fungi that are digesting decomposing stuff in your soil, including grass clippings, leaves, and any other nutrients. They are completely harmless 99.9% of the time. The fungi are always present in the soil, hidden and mostly dormant, having extensive "root" networks that can extend tremendous distances, even several miles long. When conditions are perfect (temperature, humidity and rainfall), they shoot up the fruiting body (called a basidiocarp) that releases spores into the air, then dies and quickly rots. These usually only appear for one day at a time (per each basidiocarp).
Are you experiencing a particular problem with these mushrooms, or do you just find them ugly? You could try to chemically kill the root network with a fungicide applied to the soil, but that will also kill off other beneficial microorganisms and only delay the inevitable decomposition of whatever these mushrooms are eating, and the mushrooms will eventually recover and recolonize your yard. Using chemicals could also upset the healthy, natural nutrient cycle taking place in the soil, potentially causing other problems like nutrient deficiency and pest invasion.
As with a lot of garden problems, my best recommendation is to let them do their natural thing, then die off. Once they decompose the food source, or conditions change, they will go back to dormancy underground. You get to avoid work in the yard, and nature gets to do its thing. Just sip a cup of coffee on the porch while you marvel at the wonders of nature happening in your yard.
Post a sign that says "Please Do NOT Pick These Mushrooms !!!!"
Mushrooms feed on decaying matter in the soil like old tree roots or good composty soil. The easiest thing to do without resorting to chemicals is to just rake them and let them dry up.
What Houston is suggesting will work. People always ignore signs.
However, since you mentioned that you have so many mushrooms, do like what ed21 suggests: use a square shovel or a rake to knock them off their stems.