The best way to make it impossible for him to draft of you is to draft on him instead. You will then probably just end up taking turns. Competitive bike riding is not only about who is the best, but also who is the smartest and who has the best social skills. Or who - indeed - can act the 'meanest' as you mention this word.
However I personally think you should learn to take this 'mean' out of the equation. The rules are simple and known beforehand: cross the line first and you win. Of course there is still sometimes a different 'moral' winner.
This might be why this colleague is open about his strategy early on. By being open about his tactic he wants to prevent you from feeling the beginner feeling: 'But I was better, drafting is unfair'. I think you should embrace drafting.. Don't go into a race where different rules apply to different riders.
It's great that you ask this question. Really learning to dig the physiology, strategy and tactics of drafting is to experience them. E.g. you have to train hard and then be beaten by somebody who is less strong than you to fully appreciate this. So you could just take this opportunity to learn from your colleague, even if he could be less fit than you. You will find out tactics for yourself.
I'm a triathlete myself. And drafting is one of the biggest controversies in my sport. Especially since in most triathlons drafting is still illegal. This is/was felt to be more sportsmanlike. But for professional athletes (world-class and national championships etc) most races are now draft legal. The reason being that when the sport went Olympic they discovered/realized that enforcing drafting rules turned out to be impossible when athletes levels are so close together (e.g. 50 athletes starting to cycle within one minute, so only a second apart). Also spectator enjoyment suffers from jury influence, since there's always some subjectivity/randomness to it.
A notable exception is of course the Ironman distance, where a larger and more professional jury and bigger separation during the long swim has allowed non-drafting to be kept up. Given it's history this is probably a good thing. But in practice this just means drafting is often done with the required 10 meter distance between athletes. Because at 10 meters wind advantage still exists. And considerable controversy still flares up when athletes are tempted to huddle closer together or ignore the rules. Though that's mostly in the age groups/amateur field, where there is less jury and no camera's.
Embrace drafting if you want to be like the pro's. Or look at a flock of birds in the air. They're drafting! Follow nature..
So besides tactics to prevent others from drafting, it might be even more important to draft well yourself:
- Stay less than half a wheel away from the one in front of you (unless you're one of those Americans who only learned to ride a bike later in live :P). This is the difference from having 40% advantage to 20% advantage or even less.
- If the wind is from the front left stay to the back right of the cyclist before you. Or cycle on the right if the wind is from the left
To prevent drafting of course also numerous tactics exist, but they have mostly already been given by others here.