Sorry, but what you're asking is by definition impossible. It's like asking fish not to swim, birds not to fly, or Steven Seagal not to have abs of polished steel (wrapped in thick-cut bacon). I'm not saying your guy will definitely buy you something; I'm saying you have no control over it.
Because either he's going to buy you something out of fear that you didn't really mean it when you told him not to, or he would have forgotten Valentine's Day existed unless you brought it up. Really, it's a fear-based holiday. We know you don't need candy and flowers; who needs candy and is over twelve years old? I mean, me, all the time, but I assume I'm an anomaly in that regard.
Admitting beforehand that this is a fairly cynical take on V-Day, it does seem like a lot of guys--even loving, attentive ones--think of it as a deadline underscored by threat. PROVE YOUR LOVE, says society. AND DO SO NOW. AND GIVE ME MONEY AS WELL. I'd almost call it
"rape-esque," if that were a word you were allowed to use.
Otherwise, who would buy chalk hearts? No one. No one likes chalk hearts. Anyone who disagrees in the comments is a spambot and should be ignored.
It's hackneyed advice, but I say make each other something, be it a piece of art, a poem, a mix CD, a functioning replica of colonial Williamsburg, or a planned day of fun. Share some time together. It's just another Sunday, and you should be enjoying each others' company on Sundays anyway.