any chance that you're an electrical engineer? If not, that will reduce the options.
in any case of electromagnetic interference, the general options are these:
1 reduce the strength of the emissions
2. reduce the sensitivity of the device receiving the emissions
3. change the path or environment so that less of the troublesome energy gets to the device being affected.
To reduce the emissions from the light, you might try different brightness settings. That can change the strength of the emissions and the frequencies at which it emits.
You might also try adding some shielding to the light. By this I mean some metal that reflects or grounds the RF (radio frequency) energy that hits the metal. As a means of experimenting, I recommend wrapping the light in aluminum foil. I'd wrap the whole light in foil, just to see if that works. If it does, then I'd make an opening so the light can get out and see how it does. This will also let some RF energy get out, but maybe not in the direction of the bike computer. If this works, then add a hole for the switch, etc. If it is still a good solution, go get some aluminum tape of the type used for ducts, and replace the aluminum foil with the aluminum tape.
If you are good with a soldering iron, you should get better performance by connecting the electrical tape to the light's ground connection.
Reducing the sensitivity of the device affected by emissions:
The bike computer is designed to pick up a very weak signal from the transmitter on the fork. You might try moving the transmitter higher up on the fork, in order to make the signal stronger. I'm not sure that there are any other options in this category.
change the path or environment to keep some of the RF energy away from the affected device:
This means that you need to try moving the light or computer. In its most basic form, this means moving the light away from the bike computer. Maybe try one of the brackets that let you mount lights on a fork blade??
Of course, one way to fix this is to buy a light that doesn't emit so much RF. Maybe not the most cost effective solution, but possibly the best solution.
Steve in Peoria