Quantum stasis field
A quantum stasis field prevented a Changeling from altering its biomolecular structure. 

A prototype of a device that could emit such a stasis field was developed by the Obsidian Order and tested by Elim Garak in 2371 during his interrogation of Odo aboard a Romulan warbird. The technology proved to be highly effective in forcing Odo to stay in his humanoid form, preventing him from either changing shape or from entering his liquid form to regenerate. 

As a result, Odo began to lose molecular cohesion with considerable agony until Garak finally deactivated the device and allowed the Changeling to enter his dormant state. ()

Background Information
The Romulan ship on which the prototype device was used was destroyed by the Jem'Hadar, so presumably the device was lost. It is not known whether the technology was ever redeveloped, as it was never seen or mentioned again on screen. Ronald D. Moore commented: "I think the Changelings are scary and effective because we DON'T have a weapon to use against them and I'd like to keep it that way for now. Our thinking is that the device seen in "Die" was the only one in existence."

Apocrypha
In the DS9 anniversary trilogy Star Trek: Deep Space Nine - Millennium, when trapped in the future, Odo encounters a version of Weyoun 5 whose loyalties have shifted from the Changelings to the Prophets, who briefly traps Odo using another quantum stasis field generator, although he retains enough respect for Odo to release him before Odo suffers any serious effects.

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Technology
Energy fields