If you are a manager what is the best way to communicate to a team member that their behaviour is appalling without sounding rude?

be firm but fair

Tell them Burger King is usually hiring. Many years ago, McDonalds had job applications on their place mats. I would put them in employees mailboxes.

One to one. Explain how you view their behaviour and ask them to explain it from their point of view.

Old rule is still the best. Give one positive then one correction needed then one more positive

Address the issue in a group that way it doesnt feel that you are singling them out

philasefer use tact and dignity

1 on 1 in my office.Explain the issue clearly and that you expect a change in their behavior . Perhaps they are stressed and this is how they show it.I would support and guide to resolve this.
If there is no good reason for their behaviour explain that they could be disiplined for it.

Well you're talking about assertiveness, something I haven't mastered, I either think they're not worth the energy or flip in the end

Direct and to the point.

Ask them if they think there was another way they could have handled the situation