What can be used to prevent crabapple trees from flowering and setting fruit, please? I've read that a mixture of an NAA hormone spray, such as fruitone mixed with Sevin could work, but the dosage is not known so this mixture could damage the trees.

Robert, There is a product labeled for use on crabapples to eliminate fruit, called Florel. It must be applied at mid to full bloom which is probably right now. here is a label to help. http://www.montereylawngarden.com/pdf/fruit-eliminator.pdf Read and and follow all label directions. This is not an endorsement of this particular company but intended to help in your decision making. There are probably other brand names for same product. Sevin has been used to eliminate fruit but is is very harmful to bees. Just a reminder, never mix chemicals unless they are labeled to be mixed.