My ex sent me flowers on Valentine's Day. What should I do?

For reference, we were together 2 years, we broke up last August, we haven't spoken in 4 months (we agreed on no contact for a few months), and he's convinced that we're getting back together even though I told him I'm not interested. He's very "persistent" (read: doesn't take my 'no' for an answer) and it actually took me about 4 attempts to finally manage to leave him.

Anyway, I got flowers delivered to my house on Valentine's Day, with no tag. I figured they were from my ex, since I couldn't think of any other reason they wouldn't have a note or tag on them... I am seeing someone else currently, but he doesn't know my address, so I knew they weren't from him. Anyway, I called the florist yesterday and they confirmed that my ex ordered the flowers.

So what do I do? Both my parents said I should text him "thanks" because it's the proper thing to do or whatever. And I do feel guilty that he spent money on that. But all my friends say (and I agree) that it took me enough effort to get distance from him, so opening up contact with him would just open up a door that took forever for me to close. Any time I show him kindness or politeness, it just takes even longer to shake him. All he cares about is what he wants. Doesn't matter that I've made it clear I don't want to get back together. If I text him "thanks," I just know he'll take it the wrong way.

Tl;Dr: Clingy ex sent me flowers in an attempt to get me to talk to him because he wants me back and won't take "no" for an answer.
Thank him. He did spend money and it's the least you could do.
Ignore it. You don't want to open that can of worms.
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My ex sent me flowers on Valentine's Day. What should I do?
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