You can delete your data and your customer account at any time - provided there are no outstanding orders (bookings and tickets) or current contracts (e.g. BahnCard subscription).
To delete your customer account, please log in with your user name and password. Then proceed as follows:

DB Navigator: Go to the "Login and security" area in the "Profile" menu item and then select the "Delete customer account" option. Now follow the instructions to delete your customer account.

Int.bahn.de: In the "Manage customer account" section, go to "Personal details" and then select the "Delete account" option at the bottom. Now follow the instructions to delete your customer account.