So you want to slow the Planet Down. Good idea ..
If we haven't got enough Hair brain scientist trying to do that already .
World energy consumption per year is 5.7*10^20 J. Rotational kinetic energy of the Earth is 2*10^29 J. So you could power the whole world for 350 million years until the spinning stops. If we would do it only a few decades, until fission or whatever works, the rotation wouldn't slow down more than maybe a second per day. Doesn't look dangerous to me.
OMG
First you can just pull fictitious Numbers out of a Hat.
Just slowing the Planet by a second . would cause an orbit change .
That is overly dramatic. Look into leap seconds and why we need them. The earth's rotational speed isn't constant at all: https://en.wikipedia.org/wiki/%CE%94T_(timekeeping)
That is for our time keeping . (true)
Not for the universal laws that govern the physical placement of the log spiral & helical plains that
keeps our planet in about the correct orbit . Which Albert Einstein tried hard to get scientist to
Understand .. and still don't Get it . because one needs to know and understand 7D mathematics.
Simple Example if you move 1 planet 1 degree all the others will follow at a log degree displacement .
i.e. we would move closer to the sun .. That in turn would raise the temperature of our climate by the same ratio to the Log scale so 1 degree change could be about 10c and so on .
Causing a chain reaction of equal and opposite damage .
No I am not going to explain these physics & how they works .
One reason why the world governments Don't tell people the WHOLE story .
That includes NASA . Too Much info is dangerous .
So diversionary theory's are created to distract G.P.