Bended ballscrews. What i can do?

[m_c]

I think you just need to bend them further, to not spring back. You are probably not applying enough stress to go past the material's elastic limit.

This.
If you don't bend past the elastic limit, then they will just spring back, regardless of how long you leave the clamp on. Position of support blocks, and pressure application point depends on the bend. How much pressure is very much trial and error. Apply pressure, release pressure, check to see how much things spring back, then repeat with more pressure if needed.