Mary, most of the rooting hormone products use talc as an inert base. The label on many of them state that the product is Not water soluble, so adding water would not help.
I see two options. Wearing gloves and face mask (to prevent inhaling dust) you could place the block in a plastic bag and crush it (tapping with a hammer) and return the powder to the container, or
When BR has the next hazardous chemicals (paint, fungicide, pesticide, etc) disposal collection, take the bottle to them for safe disposal. DO not place in your trash can or try to flush it down any drain.
Go to google and enter the name of your product (Greenlight, Miraclegrow, rootone, etc) and add the letters MSDS behind the name. That will give you the Material Data Safety Sheet that covers all risk/hazard info and use/disposal info.