don't knock yourself out

and I don't mean maybe

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

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.

don't hold your breath

Don't wait. Said cynically to suggest that what has just been mentioned is unlikely to happen soon or at all.