I was at a seder in which there were several non-religious people invited. The home where the seder took place is strictly kosher and had made sure to remove all chametz and performed biyur chametz, etc. After burning the chametz, we say that the chametz in our possession is to be considered like dust. One of the guests brought a bottle of kahlua as a gift. (Kahlua is a liqueur that during the year is kosher if it has a hechsher but is not kosher for passover at all.) What should someone do when a guest to a seder brings a gift which is not kosher for passover? I am unsure if kahlua is actually chametz or merely not kosher for passover. I know that refusing the gift could be humiliating to the guest (which is said to be tantamount to killing the person) but acceptance of prohibited items on pesach is also very problematic. This occurred at the first seder which was shabbos. Does that change anything? How should one respond to this situation if it ever occurred again? Kahlua is likely questionable chametz (and questionable kashrut in general), what would have happened if someone gave a bottle of scotch or a cake which would be more clear examples of chametz?
What a tricky situation! I can only imagine how terribly troubled you must have felt. Often one has no choice but to try in the nicest way to make the best of a bad situation. Here is a summary of the laws involved -
All chametz if of course forbidden on Pesach, and if found during the festival must be destroyed (Shulchan Aruch 446). Chametz that is found, or comes into your possession on Yom Tov is muktzah, and cannot be moved on the festival day, let alone destroyed (by burning or pouring down the drain). On Yom Tov you have to place a vessel over the chametz, in order to remind yourself that it is forbidden, and stop accidental use of the chametz. [There is room to say that if the chametz is still in your hands when you realize that's it chametz, you are allowed - and obligated - to take it straight to the toilet and dispose of it there. See Biur Halacha ibid "she lo yochal". But as many forms of chametz cannot be flushed down our toilets and one is still going to have to revert to covering the chametz or at least the container (the bottle), it seems to me that one should just cover the entire chametz after putting it down and destroy it after Yom Tov.].
This being so, if they gave you the gift, and on examining it you realized it was chametz, then the best thing to do is to put it down somewhere and put something over it until straight after Yom Tov, when you should immediately destroy it (in this case it being a drink, pour it down the toilet). You may be able to do this without causing your guest any embarrassment - but that of cause depends on how good you are at controlling your surprise ["You brought WHAT??!!! How dare you!!!" is probably the wrong thing to scream in such a situation].
However, all this is true only after you have received the gift. If the person has not yet given it to you, and you see what it is in their hands, the situation is trickier. If the person is a non-Jew, one should quickly and politely explain to them that as a Jew you have to be very careful about accepting gifts of food on Passover - and say "thank you so much for the thought, but it would really be so nice if you could just hold onto that yourself". Then try and direct them to returning it to their bag (or give them a bag) and let them hold onto it for the rest of Pesach. Do not in any way intend to accept the gift.
If the person is a Jew, it is even more tricky. You are not allowed to accept the gift - but they too are obligated to destroy it. So you have two options. If at all possible, and you think they will listen, then you should explain to them that chametz is forbidden on Passover, and that the correct thing to do is to destroy the chametz, which can be done straight after nightfall the next day. Then have them put the chametz down, and cover it with something until after Yom Tov. You will have to have a lot of tact and good manners to do this without hurting anyone's feelings, or insulting anyone. Humour is usually a good idea - "Wow, this is fantastic! I've never had the opportunity do this incredible mitzvah on Pesach before. We are soooo lucky - hey, do you want to do this together? You put it down, I'll cover it up, and if the bomb squad doesn't come and blow the whole thing up before tomorrow night, I'll just nuke the stuff myself!".
If, on the other hand, you know the other Jew is not going to listen to you - and will perhaps be so insulted or hurt that it may totally turn them away from Torah and Mitzvot, then my advice is to take the gift, saying to yourself that you have no intention of acquiring it at all. Then immediately take it away, cover it with a vessel, and destroy it after Yom Tov is over. Under no circumstance may you actually knowingly accept a gift of chametz.