Apple - How can I keep Safari from re-opening my tabs from my last session?

Quit it with ⌥⌘Q or run in the Terminal

defaults write com.apple.Safari NSQuitAlwaysKeepsWindows -bool false

This behavior is the resume feature.

Disable using Terminal

If you only want to disable resume for safari, type this in the terminal:

defaults write com.apple.safari ApplePersistenceIgnoreState YES

If you want to set the resume feature again, simply write NO using the same syntax.

Disable using TinkerTool

This application gives you access to additional preference settings and allows to activate hidden features in the operating system.