... as with turning off the "preview" option -- which is restricted to those with a certain experience level? I think that's Friar, but too lazy too look it up, since the point is that one who has some significant experience may well have a vested interest in keeping PM useful... and the Monastery neat and tidy.
So, I like Ambrus modification of the OP and first reply...
as I like Joost's suggestion that we use css to make the warnings more prominent.
However, I think Ambrus' concern that some folks (/me included) ignore stock warnings is best dealt with by (1) making the warnings specific and content-driven (the generic warning appears in EVERY preview, regardless of whether a warning is relevant) and (2) the Roy Johnson suggestion (at Re^2, well down in the thread) as augmented by Whitehawke's (tongue in cheek?) suggestions for adaptations of 'fortune' (AKA 'cookie').