Chriz,
If the above does not work, this most always does.
1. Go to Computer/Applications/Utilities/Activity Monitor
2. Once Activity Monitor is launched, go to the menu bar and select Window>Activity Monitor (Command+1)
3. At the top of the Activity Monitor window, in the drop down menu, select “All Processes”
4. Scan down the list of processes until you find the Flickr application.
5. Click on that once to highlight it, then click on the red stop sign (top left).
6. When presented with the alert window, click on "Force Quit".
HTH and