Canada has a sustained feral cat population that is not going to disappear. You will not be able to completely eradicate them no matter what anyone tells you.
Any removed cats will be naturally replaced by new cats within days to weeks.
Remember that if you handle the situation in an inhumane manner (extermination or exile) you are not only acting cruelly but are also compromising your company's reputation if information and/or photos leak out to the press.
1. Contact the local veterinary municipal authorities and initiate a TNR (trap neuter release) program (see http://en.wikipedia....p-neuter-return). Don't release the trapped cats back onto your premises, but anywhere nearby outside of your fence. This will contribute to maintaining a controlled and constant population in the vicinity of the plant.
2. Observe strict housekeeping procedures in the yard: closed rubbish bins, no stagnant water, no junk. In other words, make sure the yard is unattractive for cats.
3. Educate employees never to feed the cats on the premises. You might promote a feral cat feeding program in a designated place in the neighborhood, away from the plant and get people involved in feeding them there.
4. Most importantly - make sure the plant is completely proofed against pest entry. As long as the cats don't enter the plant, they pose no real risk.