Just STOP IT!
Palms should NEVER be pruned like this. Not only does it make them less wind and cold tolerant, it robs them of food (fronds produce food), stresses them out, and intensifies potassium deficiency. It can also eventually kill them.
Potassium deficiency (http://edis.ifas.ufl.edu/ep269) causes premature leaf death; therefore, K-deficient palms are unable to support a full canopy of leaves. Thus, most palms in Florida only have half of their normal number of leaves prior to pruning.
When a palm tree has all the potassium and other nutrients it needs and has a full 360 degree canopy of leaves, it is possible to remove as many fronds as will be replaced between pruning intervals, never prune above the horizontal line, and never have a dead or nutrient-deficient frond.