Preventing that this dialog appears seems not possible at the moment.
But you can define a 'custom operator' and 'create a shortcut' see: Create Shortcuts for Particle Edit Brushes?
For daily work I've converted this into an Add-on, which creates the shortcut automatically:
Install the Add-on via File > User Preferences > Add-ons. Once it's enabled you can press Shift+P for Set Parent to Object and also Shift+Alt+P for Keep Transform operation in Object Mode:

parent_shortcuts.py
bl_info = {
"name": "Parenting Shortcuts",
"description": "Parenting Shortcuts",
"author": "poor",
"version": (0, 0, 1),
"blender": (2, 75, 0),
"location": "3D View",
"category": "3D View"
}
import bpy
# operators
class SetParentToObject(bpy.types.Operator):
"""Tooltip"""
bl_idname = "object.set_parent_object"
bl_label = "Set Parent to Object"
bl_options = {'REGISTER', 'UNDO'}
def execute(self, context):
bpy.ops.object.parent_set(type="OBJECT")
return {'FINISHED'}
class SetParentKeepTransform(bpy.types.Operator):
"""Tooltip"""
bl_idname = "object.set_parent_keep"
bl_label = "Set Parent to Object (Keep Transform)"
bl_options = {'REGISTER', 'UNDO'}
def execute(self, context):
bpy.ops.object.parent_set(type="OBJECT", keep_transform=True)
return {'FINISHED'}
addon_keymaps = []
# register
def register():
bpy.utils.register_class(SetParentToObject)
bpy.utils.register_class(SetParentKeepTransform)
# handle the keymap
wm = bpy.context.window_manager
kc = wm.keyconfigs.addon
if kc:
km = wm.keyconfigs.addon.keymaps.new(name='Object Mode', space_type='EMPTY')
kmi = km.keymap_items.new(SetParentToObject.bl_idname, type='P', value='PRESS', shift=True)
addon_keymaps.append((km, kmi))
kmi = km.keymap_items.new(SetParentKeepTransform.bl_idname, type='P', value='PRESS', alt=True, shift=True)
addon_keymaps.append((km, kmi))
# unregister
def unregister():
for km, kmi in addon_keymaps:
km.keymap_items.remove(kmi)
addon_keymaps.clear()
bpy.utils.unregister_class(SetParentToObject)
bpy.utils.unregister_class(SetParentKeepTransform)
if __name__ == "__main__":
register()