The age old saying of "the customer is always right" will always ring true, however there can be times where the customer is genuinely, flat out, wrong.
You shouldn't be in the mindset of offering "good enough" customer service, you want to try and exceed the "good enough" level and break through to the "exceptional" level, if you can!
If your customer is demanding support outside of your SLA then all you need to do is (politely!) point them in the direction of the agreement and (politely!) explain why you cannot deliver their requests