As to the RadSpinEditor itself, we have always striven to keep or controls to be similar to their standard WinForms counterparts. That is why we implemented the same behavior in RadSpinEditor like the one in the default NumericUpDown control. When its ReadOnly property is set to true it disables the editing by using the keyboard, however you can change the value when using the mouse wheel as you have noted, and by using the up and down buttons. I hope this helps.
In case you need further assistance or any other question, we will be glad to help.