Non-hydraulic cement will not set if it can't exchange water vapour and CO2 with the environment (or if water is replaced). Several oily and gelly substances exist that will form a thin film above or on the surface of a liquid and slow down evaporation and CO2 diffusion. "Slow down" is not the same thing as "prevent", though. To prevent it completely, you would need a film with the thickness and properties of clingy film, which would be really noticeable.
Hydraulic cement will not be hindered by this, since it is self-reacting. For it, you would need some substance capable of blocking calcium silicates from reacting with water. Perhaps some soaps could do that. Or some lubricants could make the sludge appear not to be setting; it would rather collapse in a sort of useless gritty gruel. I know of none that would be water-souble though, or survive in the alkaline environment. Also microencapsulation of cement particles with hydrophobic compounds would work, but here we'd be talking about high-tech sabotage; also, the powdered cement properties (color, weight, fluidity, texture) would likely change drastically. Same goes for "corning" and "glazing" the cement, if possible: it would slow penetration of water and the setting reaction.
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I had forgotten this question, but by chance I've people pouring concrete just round the corner, and I asked them. It turns out that there are lots of substances that will do what you seek; they are called setting retardants, and they act by slowing down the calcium silicate hydration reaction. After a few Google enquiries I'd go for sodium glucoheptonate - on the usage directions it says that using an excessive quantity can "kill" the set of concrete. This is "commonly" done if for any reason it is impossible to empty the concrete drum on a mixer; killing the set wastes the concrete, but allows salvaging the mixer drum.
On a similar product (sodium gluconate), some producers claim that it can prolong "workable time" (which I assume means before setting) to a few days. The quantity needed is below one percent.