Opposite of "to put a good word in for"? "Backstab" doesn't work

Opposite of "to put a good word in for"? "Backstab" doesn't work

I know that when I have an associate who I think highly of and is very
capable of performing the job (or person for a relationship) [s]he is
pursuing, I will want to find the "recruiters" and put a good word in for
my friend/associate.
What if I want to make it tough for someone to get a position? I would
want to go to those "recruiters" and convince them that my associate sucks
and is not cut out for it. I would be dissuading those recruiters. But
what word would fit "I would be _ my associate"? Backstabbing doesn't
really cut it for me.
The same word would work in this sentence: "I feel my current bosses don't
like me, and I'm afraid that they'll __ me to any prospective employer."
or this one: "People think that I'm a slob that they would probably __ me
to that hot girl I want to pursue."
I'm tempted to make up a word - decommend - the opposite of recommend
where the direct object is that poor person you would be putting a bad
word in for.