I invited a non-religious guest over for a meal. I insisted that they don't need to bring any food with them. But they said they're going to bring a salad. I told them it isn't necessary but they insisted. How do I tell them politely that I can't eat their salad? Preferably before they make it. Or when they arrive? (They have heard of Kosher and think their salad is Kosher, but assume the salad for whatever reason isn't kosher.)
'Judah, the son of Tema, said: 'Be bold as a leopard, light as an eagle, swift as a deer, and strong as a lion, to do the will of thy Father who is in heaven' '. ''Bold as a leopard'' means that a man should not be ashamed in the presence of (other) people who may laugh at him during his service to G-d, blessed be He.
I suggest that you explain that many vegetables contain insects which we are forbidden to eat. There are special methods of removing these insects and generally we do not eat salads that have not been prepared according to these guidelines.
I've had similar situations with a guest bringing me a food present that "seemed" kosher. I've usually handled it by being honest without going into too much detail.
Explain to them what your kosher requirements are. You also need to be clear and a bit emphatic that while something may be considered "kosher", there are numerous interpretations and variances to each person's custom and definition of kosher. Cite one or two specific examples esp. if it applies to their salad, as to why you cannot eat it. E.g. - do you require that you personally inspect all veggies under a light even if the veggies have been "bodek" - checked before packaging? (I'm not stating that this is necessary, but if that's what you do, and they don't, you tell your friends that.)
Ideally, state this before they come, as this will avoid toil and embarrassment. It also lets you suggest a specific packaged item that they CAN buy, if they insist on giving you a gift. After they have brought the food, it's a bit difficult. I know that as your question states, you've told them not to bring anything, but they seem well-meaning, as they didn't understand the rules.
Most good friends will understand and respect your dietary requirements, and may even be fascinated by it. If they get offended, seriously, think over the friendship, itself. It indicates that they are intolerant and possibly, imposing.
When I invite guests, I ask them, before they come, if they have any restrictions - dietary or otherwise. I go out of my way to accommodate the "stricter" level where possible. It's called being a good, gracious host. Likewise, if your friends get offended, and you want to attempt to reason with them, you can mention what you do to be a gracious host, and it's only sensible for them to be "gracious" guests. It sounds like a "harsh" tactic, and I would use it only when absolutely needed. Sometimes people need a bit of a "polite shock" to wake them up.
A simple forthright explanation is best. Being gentle in tone, you can explain that even if the vegetables are kosher, the knives and other utensils used to chop the vegetables and prepare the salad may have been used on other non-kosher items. And any non-kosher residue those utensils might have would make the salad non-kosher.
If they say, they will buy a brand new knife and cutting board in order to prepare the salad for you, you can tell them the knife has to be toveled in a mikvah before used to chop the salad. That will either be a huge inconvenience for them, and a deterrent that will lead them to drop the matter entirely. Or it will stimulate them to ask questions, and possibly go to a mikvah to tovel the knife. That is one heck of a kiruv opportunity. So either way this leads to a win win.