Short answer
Block the searches that can never become work: jobs, courses, salaries, apprenticeships, parts, wholesale and anything starting with how to. These are charged at emergency plumber rates and come from people who were never going to book you.
Every plumbing ad account leaks money into searches that could never become a job. The leak is quiet, it is charged at emergency plumber rates, and most accounts I open have never had it closed.
What is actually happening
Unless you say otherwise, Google shows your ad for searches it considers related to what you bid on. Related is doing a lot of work in that sentence.
Bid on plumber and you can end up paying for plumbing apprenticeships, plumber salary, plumbing supplies and how to fix a dripping tap. Every one is a real click at a real price from somebody who was never going to ring you.
The starter list
Add these before you do anything else. They apply to practically every plumbing business:
People who want to be a plumber, not hire one jobs, job, career, careers, salary, wage, apprentice, apprenticeship, course, courses, training, qualification, college, nvq
People buying parts rather than labour supplies, wholesale, merchant, screwfix, toolstation, spare, spares, parts, buy
People doing it themselves how to, diy, tutorial, youtube, video, guide, fix my own
Not your market free, volunteer, charity, salvage, scrap
That list alone usually removes a meaningful slice of wasted spend in the first week.
Then the ones specific to you
This is where the real money is, and only your own account can tell you.
If you do not do gas, block gas. If you do not touch commercial, block commercial, industrial, factory and warehouse. If you do not do new builds, block those too. Every service you do not offer is a source of clicks you will have to turn down.
The same applies to places. If you will not drive to a particular town, it is better to block it than to pay for clicks from it and then explain you cannot come.
The habit that keeps it working
Open the search terms report once a month and read what people actually typed. Not the keywords you bid on, the real searches.
It takes ten minutes and it is always revealing. You will find things nobody could have predicted, because people search in ways nobody predicts.
Anything that could never be a job gets added as a negative. That is the whole routine, and an account with that habit costs meaningfully less per booked job than one without it.
Where people overdo it
Blocking cheap and price. It is tempting, because nobody wants a price shopper. But somebody searching cheap plumber near me at 8pm with a burst pipe is still a job, and they are not really price shopping, they are worried.
Block things that can never be work. Do not block things that merely sound unappealing.
Why this is not just admin
An account left alone drifts. Google keeps finding new searches to show you for, and most of them are worse than the ones you chose.
A plumber doing this monthly and one who set the account up two years ago and never looked again are paying very different amounts for the same work. This is a large part of what managing an ad account actually means, and it is also why a flat management fee matters: trimming your wasted spend does not cost me anything, so there is nothing stopping me doing it.