Several ways I can think of, some of them heavy-handed and/or cheesy:
1. Trap them in a room with strong walls and toss in a continuous (as in "save every round") Dex poison.
2. Send them against a group that could easily TPK the party, but they use subdual attacks (perhaps with a custom metamagic feat to make a spell do subdual damage).
3. If the purpose of the kidnapping is to force them into a particular mission or a particular area, all this can be more easily done by blackmail or hostage situations (less effective if they PC's have a "kill everything and raise the hostages later" attitude).
4. For the true Rat Bastard: kill them all, and ask each one if they accept a Raise Dead spell. If they refuse, their comrades can't raise them either. If they accept, they are not only captured, but short an experience level!
Every D&D player I have played with simply prefer death for their characters to capture. So you might need to tell the PC's that their capture is necessary for the scenario you're running, and that their characters can be enriched by the experience. Then, of course, you will need to carry out your promise.