Validation rules in conjunction with a roll-up summary field can be used to prevent users
from adding or deleting records. To create a validation rule: 1. Build a roll-up summary on the parent object that sums the number of child records. 2. Create a validation rule on the parent object that conditionally prevents changes to the number listed in the roll-up summary field. If a user tries to add or delete a record, the validation rule prevents the user from doing that.