A lot of people are not mature enough to hear this. The ideal outcome of meeting someone you dont like is that person changes over time to be better, hurting them just drives them further away from you.
You dont “fix” an abused dog that barks at everything by hitting it.
If you want a poly relationship that works it has to start poly. It is very common for someone just getting into poly to hurt a nonpoly persons feelings by trying to be “suddenly” poly.