As it appears you can still log in;
1. Change your password https://www.playstation.com/en-gb/get-help/help-library/my-account/login-details-and-verification/ps4-forgot-psn-password/
2. Set up 2 Step Verification and sign out all active log-ins - https://www.playstation.com/en-gb/explore/playstation-network/two-step-verification/
3. Deactivate all devices remotely - https://www.playstation.com/en-gb/get-help/help-library/my-account/device-activation-deactivation/remote-deactivation/