Disconnect the Mac from Wi-Fi and Ethernet
Quit suspicious apps from Activity Monitor
Restart the Mac in Safe Mode
Remove suspicious login items and startup items
Delete unknown apps from the Applications folder
Empty the Trash
Remove suspicious browser extensions
Clear browser history, cache, and website data
Check and remove unknown configuration profiles
Update macOS to the latest version
Run a reputable Mac antivirus or anti-malware scan
Remove detected threats and quarantine items
Change passwords for important accounts
Enable two-factor authentication
Restore files from a clean backup if needed
If problems remain, erase the Mac and reinstall macOS