Prevent WPF CheckBox from checking/unchecking with the +/- keys

I am creating a form in WPF. Some fields are optional, but they're implemented as such that, their TextBoxes are inside CheckBoxes. Everything seems to be working ok, except that, if the user tries to type in a + sign or a - sign, the checkbox gets checked/unchecked automatically, and the character is not being written to the TextBox.

I would assume that it is because the CkeckBox listens to key presses while it or its content have focus, and it may be probably wired up to behave like this by default.

Is there any way in which I can prevent the checkbox to listen to these 2 keys?

(I am not posting the sample code, because it is just a TextBox inside a ComboBox in WPF)