How to restrict value on form

[SupportCenter]

is there anyway of only allowing one response for a customized field?

[Klemen]

The tweaks you are asking for (in several posts) require PHP coding. I would recommend that you try hiring a programmer to make the changes exactly as you want, for example by posting a project at www.rentacoder.com

I apologize, but I simply cannot provide solution for every customization people ask for.