Foreach site-collection you've used the Taxonomy Extension you have to click on the root-site at "Site Actions"->"Site Settings".
Click "Site Collection Features" on the right side.
Search for "SharePartXXL Taxonomy Extension" inside the list. Press the "Deactivate"-button and confirm.
Repeat for each corresponding site-collection!
Please select the tab "Operations" at the SharePoint Central Administration site. Click on "Solution management" at the Global Configuration
You should find "sharepartxxl.taxonomy.wsp" inside the list, please click on this entry to see its properties.
Please click on "Retract Solution"
Make sure you've chosen both "Now" for the timer job and "All Contents Web Applications" as the retracting scope.
Confirm the uninstallation of the Taxonomy Extension for Microsoft SharePoint by the OK-Button. Now you'll return to the solution-list from the last step.
After a short period of time the solution should have the status "not deployed" within the list (you can refresh it's status by pressing F5).
If so, click on the entry and finally remove the solution by clicking on "Remove solution"