"You must be joking" is the response when they need something in return; try offering something - credits, preferably, but they'll take technology, ships, starbases, resources, whole colonies, whatever. Also, consider bribing their neighbors to declare war on them; your enemy might divert forces away from you to fight their new enemies for the next fifty turns, and may make them more amenable to making peace.
When I play, I have tech trading turned on but players can only trade tech they've discovered, so I make a point of giving away farming and economic technologies to minor races which will increase their raw production - then I bribe them to make war on my enemies, and it's cheap. Actually, I play the role of Galactic Loki - if I'm not fighting another race I bribe them to fight their neighbors. It permanently damages their relations with each other (so they're less likely to ally against me), it keeps them busy, and saps their resources.