How do I politely reject guys when I have a boyfriend?

I'm a people-pleaser and feel bad rejecting guys even though I have a boyfriend because I don't want to hurt their feelings. I usually get their numbers and text them that I'm 16 (the boyfriend excuse doesn't always work- scummy guys exist), and then I block them. I want to be more upfront with people because constantly pleasing everyone is stressful and complicated. How do I politely reject guys?
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Guys- what is the least hurtful way to get rejected?
How do I politely reject guys when I have a boyfriend?
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