"Suspicious Activity Might Have been Detected" malware in Safari or other browsers
Here's the solution on iOS/Safari:


Close Safari
Open Settings
Look for the Safari settings, click it
Click Advanced (at the bottom)
There is a setting for JavaScript, turn it off
Close Settings
Open Safari
Close the message box if it is still open
Click on the "show me all the web pages" icon in the lower right (whatever it's called)
Close the page in question
Go back to Settings and turn JavaScript back on


All done!