I have had this "treatment" many time, both with an enema in the gut and without. In some ways the insertion is almost as good as the spanking. When the butt plug starts pressing against the anus I can almost prematurely ejaculate. As the slippery shaft slides on in and the dilation stars and that little twinge of pain starts from the streaching I push back against it with expulsion muscles which makes it easier to take.
Then "hump day" occurs as the largest part slips by the sphincter, it almost the sensation of going down a rollercoaster, but it is over way too quickly.
We do not use heavy paddles for these spanking as the force of hard lick with a heavy paddle may rear the anus leading to a fissure or do other damage. A light otk type paddle is suitable. Other instruments that work well are canes, floggers, and the best, a strap. The strap of course gives a good ass burning but if it hits the plug will give a deeper almost pleasant sensation as the plug pushed the anus in then lets it come back, almost like an ass fucking. Another pleasant thing that happens is the plug moves back and forth across the prostate,(Sorry ladies, one of the few good things about having a prostate.) I was inadvertently milked this way once, of course the spanker did not stop and the rest of the spanking was not as joyous an occasion as the first part.
If the spanking is given with an enema onboard, my preferred way of taking a spanking with a plug up, there is an even bigger sensation, the force of the swats are transferred to the water and it almost feels as you are getting light spanking from the inside.
Caution do not over distend the Colon with the pre plug enema and do strike the base of the plug with force from a heavy object. Also give the spanking all over the buttocks and especially the sit spot and the very upper things, just don't go too far down the the thighs un less you are using a very light stingy instrument.