How can a Dragon keep humanity in check?

I am a Dragon and I have an infestation problem in my world. Currently I am the single one of my kind and your representation of a dragon in movie Desolation of Smaug is fairly accurate only there's no such thing as 'magic'.

All of my life I had enjoyed an indolent life style: hunt when I am hungry, sleep when I am sleepy, and ponder on the meaning of life when I've got nothing else to do.

I don't remember when the humans first appeared but they've been around for some time in one form or another (I had tried to keep the count of passing seasons at some point but lost interest after several thousand cycles). However a couple centuries ago I stumbled upon an amalgamation of humans they'd called a "Kingdom". There I found an interesting contraption(some kind of torsion powered spear thrower) that I suppose was designed with a sole purpose of bringing me down from the sky.

Needless to stay it utterly failed. In a fit of rage I burned down all hummies and their dwellings in sight for such insolence. But then I noticed a disturbing pattern: from the earliest memories of these creatures that I have, I remember them still hairy as your average wolf throwing stones at other animals. Then they started throwing sharpened sticks instead, and then using some primitive yew devices to launch those. There is a trend: they progress and their ability to kill other beings improves over time.

Back then I thought that I can easily solve the issue by exterminating them completely. I had burned every human being in all of the "Kingdom" and then continued doing so through the rest of the continent. I then doubled back and flew to the next continent over the Great Sea where I found more human amalgamations(I still wonder how they've managed to get there - it takes a week of continuous flight for me!) and burned them down too.

I took my time exploring the continent(I hadn't been there for a very long time) and noticed that some of the meals that I liked from before were now gone and I knew whom to blame. I checked the other continents and found more humans on every single one of them. I wiped them all out.

Then when I was done with my campaigning I returned to check on the "Kingdom" and found a big amalgamation of humans numbering in thousands not very far away(a day or two of flight). I burned them down. I scoured the continent all over again and found more humans...

I had spent the next two centuries campaigning all over the world several times over. But every time when I leave a place it gets infested with humans in a matter of decades. They survive in small bunches and then multiply like crazy. At this point I don't even know if it's worth the effort to keep going like this. Long gone are the days when I basked in the sun leisurely without worrying what region should I cleanse the next moon.

I must admit that these creatures seem to be only slightly inferior in terms intelligence. After all they are the only other creature in this world capable of harnessing fire. I also have to admit that I should've exterminated them when they only inhabited a single continent(or just limit the infestation there).

Here is my question: How can I keep these hairless apes at bay and prevent them from inventing new ways to kill(specifically me). Culling seems to do the job, but it is very tedious and time consuming. Is there a way how to solve the issue permanently or at least optimize the process?