Even though it has been said several times already, this point is important enough to repeat—alignment is a tool to aid role-playing, not a hammer to force characters to do things they don't want to do.
The DM should never tell a player, "Your character can't do that because it's against his alignment," unless that character is under some type of special magical control. Let players make their own decisions and their own mistakes. the DM has enough to do without taking over the players' jobs, too.
Despite this prohibition, the DM can suggest to a player that an action involves considerable risk, especially where alignment is concerned. If the player still decides to go ahead, the consequences are his responsibility. Don't get upset about what happens to the character. If the paladin is no longer a paladin, well, that's just the way things are.
Such suggestions need not be brazen. True, the DM can ask, "Are you sure that's a good idea, given your alignment?" He can also use more subtle forms of suggestion woven into the plot of the adventure. Tomorrow the cleric intends to go on a mission that would compromise his alignment. That night, he has a nightmare which prevents any restful sleep. In the morning he runs into an old soothsayer who sees ill omens and predicts dire results. His holy symbol appears mysteriously tarnished and dull. The candles on the alter flicker and dim as he enters the temple. Attentive players will note these warnings and may reconsider their plans. If they do not, it is their choice to make, not the DM's.