What we could suggest you is by using the ChildrenOfType method to find the TextBox placed inside the Template of RadNumericUpDown and set its IsUndoEnabled method to false.
Please, check the attached sample that demonstrates that.
Hope this helps.
Regards,
Nasko
Telerik by Progress
Do you need help with upgrading your AJAX, WPF or WinForms project? Check the Telerik API Analyzer and share your thoughts.