Removing "Powered by Strikingly" footer is a Pro account feature. Check out the Pro Guide!
Once you have upgraded to Pro, you can remove it by going to Settings > Advanced> Header & Footer> and unchecking the "display Strikingly footer" box!
If you have upgraded but you still see mysite.strikingly.com when you visit mysite.com, you have connected your domain incorrectly. See checklist.