Are end-of-term student evaluations taken seriously? My question is in the title, but I'm specifically curious about the case where all students give a professor a negative evaluation. In that instance, does the department/university stop giving him new courses? Does it have any effect whatsoever?
It depends on the university, the class dynamics, and the position of the professor/lecturer (i.e. are they ajuncts or casuals? Or are the tenured/continuing contracts? Are they solely there to teach, or is teaching a small component to their research expectations?) 

Short answer: Generally not. Student evaluations are taken with a large cup of salt. This is because the anonymity of evaluations allow for students to provide unhelpful, derogatory and discriminatory comments without repercussions. If there is trouble with a class, the professor/lecturer may have already brought it up with the head of school/faculty prior to the course ending or evaluations coming through. 

I would also be careful in claiming that ALL students will complain. You might find that some students are only agreeing with the loud voices who are disgruntled, but may not feel the same. Students also often tend to only provide bad evaluations, students who really enjoy the course do not always tend to indicate as such on student evaluations and this is taken into consideration. We are more likely to provide negative feedback when we are displeased, than we are when we are happy. 

Women and individuals of colour/disabilities and so on are more likely to receive bad evaluations based on preconceived biases that students may not even be aware of. These are also taken into consideration when evaluations are received.

Student evaluations can be helpful regarding what the professor can do to improve the course (structure, content and so on) but they are not helpful in assessing the popularity or whether students like a professor. It's not a professor's job to be liked by the students nor is it a popularity contest. 

The best teaching evaluations are those where students have carefully considered their responses and provided constructive feedback for improvement. Anything that reads as malicious or aggressive is ignored, and anything that says one thing which can be contradicted with evidence (i.e. a student claiming that a professor didn't spell out expectations, where in fact, there is hard evidence supporting that the professor did) will be ignored as well. If a professor received contradictory evaluations (i.e. one student saying really positive things, others who say exact opposite on the same topic) there is a general assumption that the student who says the negative things did poorly and is frustrated with their grade.