C# A way to throttle specific IP/identifier in wcf

Question

I have WCF service:

public interface IService
{
    void Run(long userId, byte[] data);
}

And want to throttle userId=123 requests in such matter that there will be 100ms ping between calls, rather than some exception saying "Too many". It is also not wise to just block thread requests by inoking Thread.Sleep, because I will throttle entire thread from thread pool.

How one can achieve this behavior?