I never have a problem with someone sending me a lowball offer. Smart managers are always looking for steals, and you never know if the other manager will have a brain fart, so why not give a try. I usually give two counters: one for what they offered, and one for what they asked for.
When "that guy" gets to be too much - getting pissy or whatnot - then you simply confront them with logic. Take that Castro for two players deal that the OP mentioned. Ask him if he would accept the deal if it was offered to them. If he say no, then you point out he just admitted to trying to rip you off. And if he say yes, then ask him why he wants to trade the Castro for two lesser players in the first place.
"If you wouldn't take the deal yourself, what makes you think I would be interested?"
"Thank you for being SO generous, but in the interest of competitive balance, I just can't possibly take advantage of your team by accepting your offer."
As of 8/4/13
C:Napoli 1B:Goldshmidt 2B:Kinsler, Gyorko SS:Tulo, Profar 3B:Longoria, Machado, Middlebrooks OF:Harper, J. Upton, AGon, R. Davis, Venable SP:Scherzer, Minor, Ryu, E.Santana, Beachy, Samardzja, J.Johnson (DL), T.Ross RP:Perez, Rodney, Farquar