What word means the opposite of smile warmly?

There isn't one single perfect opposite for "smile warmly". It depends on what aspect of the phrase you want to oppose. Here are some options:

Opposing the warmth:

* Scowl: A harsh, angry expression.

* Frown: A sad or disapproving expression.

* Glower: To stare angrily or fiercely.

* Grimace: A facial expression of pain, disgust, or disapproval.

Opposing the smiling:

* Stare blankly: To look at someone without any emotion.

* Be expressionless: To have no facial expression at all.

* Look away: To turn your head away from someone.

Opposing the action itself:

* Ignore: To pay no attention to someone.

* Reject: To refuse to accept someone or something.

* Be indifferent: To not care about someone or something.

The best opposite will depend on the specific context in which you are using it.