Risky business but would I be allowed to

[#1]

Put a sign up saying ,

"No people with food allergies please"

If I was to open a resteraunt.

Just thinking you must have to know every ingredient for every single item and from every supplier, of anything you use 

[#2]

You are catering to the public and it is up to you to ensure that you do not kill them in your establishment.

[#3]

That must be very hard when people go for a meal .im glad ive no food allergies

[#4]

I think you'd be safer to say "all food served in this restaurant may contain traces or larger amounts of the following allergens:

Then list all the ones that you are required to notify by law.

After all, if you cook with those things in your kitchen you could never guarantee any food would be completely free of them.