I don't know about that

and I don't mean maybe

Indicating that one is serious and not joking about a matter.

as you know

as it is already known.

don't call me sir, I work for a living

Used by senior enlisted military personnel to indicate that they are not commissioned officers.

don't call us, we'll call you

A stereotypical statement from a hiring organization to a potential candidate, suggesting that the candidate will not be hired.

don't drop the soap

Used as a mockery to someone who is about to be or should be confined in prison.

don't get someone started

Used to attempt to avoid or abbreviate a discussion that speaker or hearer may wish to avoid.

don't give up your day job

Somewhat sarcastic but usually good-natured advice used to criticise a person's degree of talent (especially in music or other performing arts), implying that they are unlikely to be successful.

don't go there

Do not broach that topic.