Workaround:

class MyTextBoxInputHandler : TextBoxInputHandler
{

public MyTextBoxInputHandler(RadTextBoxControlElement el) : base(el)
{
}

public override bool ProcessMouseWheel(MouseEventArgs e)
{
return false;
}
}

 radTextBoxControl1.TextBoxElement.InputHandler = new MyTextBoxInputHandler(radTextBoxControl1.TextBoxElement);