Stop falling for it, and force them to adapt. A move is only as good as you let them use it. As for kirby's down + b, Just wait for the fall and charge a smash in the middle of the move, once he gets out, BAM ko at 50%+ (Snake's forward smash is beautiful for this)! And since you can still throw him, just pick Dedede, falco or anyone with a throw loop, and loop the ***** so he never uses that move again. I fucking hate smash and I know this shit.
Also as a meta game, repetitiveness can be confusing at some points. I.e. say your playing against guile in SF2. He is just spamming sonic booms. Hes thrown twenty in a row without doing something else. There sometimes is this voice in your head thats like "he threw out like twenty of them, no way hes going to do it again! Hes going to bait a jump-in or poke, no way he would be dumb enough to do it again." so you walk up to hit him, and BAM!! There is a sonic boom in your grill. Frustrated, you jump in. FLASH KICK! Now your just mindfucked. And your in no shape to continue.
The 'old man' tactic is lame, but you need to think strategically against things like that. You basically have to scare them out of using their 'one move'. Make them play technical, use shenanigans, be cheap, just make them play your game. And people can only play lame when they are ahead. If you have life lead, just run away and annoy your opponent to coming to you. Learn your timing, your counters, and how to adapt to your opponent. Basically L2P scrubs.