Use the Max discount field to restrict the amount of discount that can be given on the bundle. The field is in the bundle's settings (see Where to find Product Bundles) and requires the CPQ add-on.
The maximum discount applies to the bundle's total price. Enter it as a percentage. Leave the field empty or set it to 0 for no limit.
If a bundle in an opportunity, order or subscription has a higher discount than the maximum, users who are not administrators can't save until the discount is lowered. Administrators see a warning but can still save.