You can see which previous items that have been run or are currently running on your system tray by clicking on Start >> Settings >> Taskbar and Start Menu, tick the box that says "Hide inactive icons" then click "customize". There you will see a list of Current and Past items. If you would like to remove the past items then follow these steps:
1. Start Regedit. If you are unfamiliar with regedit please refer to the FAQ on how to get started.
2. Navigate to HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\TrayNotify
3. Delete the keys PastIconsStream and IconStreams
4. Exit regedit and restart Explorer.
5. To end explorer right clicking anywhere on the systemtray and choosing "Task Manager". Click on the "Processes" tab and find Explorer.exe. Click the End Process button. Your whole systemtray should dissappear.
6. To restart Exporer, while still in Task Manager, go to the "applications" tab and choose the "New Task..." button. Simply type Explorer.exe and click OK.