Honestly, I think that owners should be just as vigilant as the mashgichim and they should do spot checking on their employees as well. Just because they pay for hashgacha does not mean that they should leave all the checking only for those times when the mashgiach comes. Owners should be as strict and as vigilant about it in their business as they are in their own home.
Not allowing outside food is one way. Supplying food for their employees obvious is part of that. Believe me, supplying the food really works. Employees are very happy to eat your food at work and save the money to eat their own food at home.
Firing employees on the spot for breaking this rule is another way to show how important the rules are. If immediate loss of employment is the consequence for breaking this rule, employees would take it more seriously. If you make excuses for employees then YOU are NOT serious about it.