A quantum stasis field prevents 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. (DS9: "The Die is Cast")
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."