My boyfriend just takes jokes too far!

My boyfriend gets pissed off when I send him "k" in a text message he told me not to do that it does his head in so I sent " k lol jks " then he starts sending me k now every time I reply and I told him alright I get it and it's annoying me now and then he sends k again. How do I deal with situations like this? Lol it's pathetic but it can be very frustrating when I'm just joking around and when I need him to be serious.
My boyfriend just takes jokes too far!
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