Well, this is my opinion. I will write about girls, because... I didn’t have to deal with the couples. What to do with guys, well, it’s about the same, they are people too.
My opinion will not be popular, but honesty is the best remedy. And honesty flavored with a light but sweet lie... even better
Do it as a compliment. Notice something outstanding about her (long gorgeous hair, size 3 breasts, gorgeous ass, blue eyes), and say that this is too much for you. Like, I’m the kind of person who likes girls with... (small tits, short hair, narrow butts, brown eyes)... h.z. you must say something that will give her a reason to think that she is better than you love. Or a universal method (in cases where outwardly there is nothing to be found there): "You are smart and I feel too stupid next to you, I can’t do that" (works almost always, the main thing is not to get caught by someone who is really smart, otherwise this nonsense won’t work), "you’re so successful that it seems to me like I’m a loser, sorry." In short, when refusing, emphasize your merits. not flaws. You won’t lose money, but the person will be pleased.
Now about your options:
1) Silently stop the correspondence - well, so-so, you can offend a person, but you don’t choose your appearance, it /they don’t deserve it.
2) Write a caustic comment about the photo you received - only scumbags do that, alas. Well, girls a la "I am a princess" - a very common phenomenon is arrogance due to the presence of a beautiful appearance (by the way, even more often, this is the female version of scum). Even if a person doesn’t turn you on outwardly, mocking him is the last thing. He/she already knows that if you are not a sex symbol, kicking someone where it hurts is bestiality. Again... For some, it may just be squeaky and chic, but you are ruining the person’s self-esteem.
3) If you write that the couple is not to your taste/age category/weight category/any other reason - you can, but not gracefully. Those. It will work, yes, but there will be a residue. Several similar refusals and hello complexes. So it seems like I don’t care about outsiders, but personally I don’t want to be the one who gives them a start.
4) My version is higher.
5) "We write something like this: "We received the photo. Thank you very much", "We won’t suit you" or "We won’t be able to get to know you." - yes it will do, the main thing is not to develop why.
Z.Y. And there is another option that is not related to the topic. To refuse girls who showed interest. Something like "you’re very attractive, I’d like to spend more time with you, but I’m married/in a relationship, sorry." It won’t offend anyone. Although it’s probably fashionable now to refuse as harshly as possible with humiliation... I don’t understand this fashion