After working 5 years as a developer it is starting to really bug me that my clients (who I work for almost as long) don't take my word for it when I say "that's not possible".

Of course I don't mean that it is impossible per se but rather that this feature costs to much time and therefore money to even research if there is a workaround for it and they would never want it, if they knew the cost. Still, even when I tell them that this way, I have to explain this over 3 meetings with 1-3 days where I am told to try it anyway (which ends with the same arguments I told them in the first meeting) while I could be doing work that really is worth my time and their money.