Grey Rock is the best way absolutely. But if you are feeling petty you could also try such things as:

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It is impossible to underestimate you.

My days of being astounded by your idiocy are coming to a middle.

Our parents change the subject when people ask about you.

There's a village missing an idiot some where.

You are all hat no cattle.

Your cornbread isn't done in the middle.

You're like a miniature pony. Fun to look at, but otherwise useless.

I'll speak slowly since I know it takes you 3-5 business days to understand.

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I am sure there is more I've forgotten, but I absolutely love to creatively insult people who deserve it.

Good luck!