I'm sorry you have to deal with this. This is coming from the opposite angle, but I can relate very much.

I follow a vegan diet but don't make it a topic, unless someone else wants to talk about it. (Even if I wanted everyone to become vegan, I think proselytizing would lead to nothing but resentment.) Ironically, at my workplace, where everyone but me eats meat, every single time we have lunch together someone will strike up a conversation about my food choice. This is always with polite intentions and nobody questions my choices, but even so I sometimes wish I could just eat my food and talk about the weather. So far, I wasn't annoyed enough to say anything, but if someone I need to get along with was regularly trying to change my dietary habits, I would say something like:

Hey, we talked this over so many times, and I feel quite frustrated with this conversation. I really don't want to change my diet, and I frankly don't care who of us is right. What I care about is that we get along in this office, and that I can make my own choices. But when you tell me that [I am a bad/silly/unhealthy person etc. for eating what I eat], I feel [singled out/uncomfortable/annoyed]. Can we please not talk about our eating habits for a while?

Chances are your vegan colleagues didn't realize how you feel about the issue. If they still don't leave you alone, you could try to escalate by appealing to their "ideology", but this might also backfire:

Listen, if your goal is to convince me to adopt a vegan diet, you are doing a very bad job. Every time you criticize my food choice, I only become more angry inside. I can really see how I am starting to think badly about veganism in general.

As a side-note, you could als ask this question on https://vegetarianism.stackexchange.com/

I'm sorry you have to deal with this. This is coming from the opposite angle, but I can relate very much.

I follow a vegan diet but don't make it a topic, unless someone else wants to talk about it. (Even if I wanted everyone to become vegan, I think proselytizing would lead to nothing but resentment.) Ironically, at my workplace, where everyone but me eats meat, every single time we have lunch together someone will strike up a conversation about my food choice. This is always with polite intentions and nobody questions my choices, but even so I sometimes wish I could just eat my food and talk about the weather. So far, I wasn't annoyed enough to say anything, but if someone I need to get along with was regularly trying to change my dietary habits, I would say something like:

Hey, we talked this over so many times, and I feel quite frustrated with this conversation. I really don't want to change my diet, and I frankly don't care who of us is right. What I care about is that we get along in this office, and that I can make my own choices. But when you tell me that [I am a bad/silly/unhealthy person etc. for eating what I eat], I feel [singled out/uncomfortable/annoyed]. Can we please not talk about our eating habits for a while?

Chances are your vegan colleagues didn't realize how you feel about the issue. If they still don't leave you alone, you could try to escalate by appealing to their "ideology", but this might also backfire:

Listen, if your goal is to convince me to adopt a vegan diet, you are doing a very bad job. Every time you criticize my food choice, I only become more angry inside. I can really see how I am starting to think badly about veganism in general.

As a side-note, you could also ask this question on https://vegetarianism.stackexchange.com/