iOS Pattern with Objective C example to prevent concurrent calls to a method

I am looking for the best pattern to adopt to ensure a method is prevented from running concurrently. An example in objective C would be ideal.

Is it best to use a lock inside the call wrapped around an if statement or an NSOperationQueue and allow only 1 concurrent operation and check the queue for items before adding new ones to the queue. Or should I be looking at something completely different?

The objective is to allow the method to be called on an object and for subsequent calls to be ignored if it is already running.