warn somebody about something
warn somebody against something
warn somebody of something

Still, Israel's leaders have been warning against such a decision.

A few men warned against it from a the beginning.

That would be to do just what she had warned me against.

He warned against such an action, saying it would be "an act of war."

Did that woman who read your hand warn you against a fat boy, by any chance?

I don't care if Min warned me against her or not!

But he warned against the application of more political pressure.

Be careful I thought, this is the sort of thing you were warned against.

It did not name the officials, but warned them against political or military activities in the zone.

First he warned against the illusion of total security for any nation.