I have deactivated him in the options:=)

You can hear him laughing and the sound of spray paint if you are in the main gas station shop area. Yeah I usually grab a bag of trash out of the dumpster or I try to keep a bucket around to throw at him.

As you progress through the game there is an "Anti Dennis" thing in the car park to the right of the gas station that will generate bags of rubbish for you.

Or empty a bin in the shop and throw that at him

I just switch him off in the settings though!

Yeah I deactivated him. If we can beat the living daylight out of him, then he doesn't need to be there.

The mechanic of "Dennis" i believe shows how the devs lack an understanding of progressive accomplishment. Lemme explain...
So when you start the game Dennis is a nuisance but you can somewhat deal with him as you work. However as you progress this starts to get more difficult as your attention is directed elsewhere so he ends up tagging your station more often.
This is actually great as it gives another challenge to the player amongst the other mechanics however....
At the "endgame" you get the "DENNIS ALARM" which gives you ample warning that hes coming and will provide you a prop to throw! Sounds great right?!....well...no....because we as the player have become fatigued with Dennis and the tedium of chasing him away. So many players felt this way that the off option for him was made a togglable which is the lazy mans bandaid.
What wouldve been cool is if you could "employ" the boy with certain products or maybe an exclusive candy currency to have him do something like say....pick customer car trunks and deposit items he steals into a chest? Hed only be active in the day shift due to bedtime.
Now, not only is Dennis not a problem but now hes an added bonus to your business....aka progressive accomplishment! The player has defeated this tedious issue and gotten something cool outta it.
The same issues are present with sand piles and uncle. I get the devs mindset of wanting to keep the player engaged but infinite tedium is not it. There has to be an endgame where most everything is automated and the player can set back and watch everything theyve built just work.