Summarize:

Click the "Go" button at the top center of the screen, then select "Utilities" from the drop-down menu. Select "Activity Monitor" on the resulting screen. Once you're in the Activity Monitor, find processes with "Chromium" and "Chromium" Helper in the name. Select both and hit "Quit Process" for each. If an "Are you sure?" tab pops up, select the "Force Quit" option. Clock the "Go" button again, then select "Go to Folder." Enter "/Library/LaunchAgents" in the search bar to find and open the folder. Locate "org.chromium.Chromium.plist" in the folder and move it to the Trash. Once you've done this, search for "~/Library/LaunchAgents" in the same manner. Send the "org.chromium.Chromium.plist" you find here to the trash as well. Finally, search for "~Library/Application Support" in the same way. Trash the "Chromium" entry you find there. Click on "Go" once again, then choose "Applications." Find the entry for "Chromium.app" and right-click it. Select "Move to Trash." You may need to enter your Mac password in order to complete this task. Click on the Apple logo in the top left corner of the screen, then select "System Preferences" from the drop-down menu. Select "Accounts" and then the "Login Items" button. Highlight "Chromium" on the list that appears, then press the minus ("-") button at the bottom of the tab. This will prevent any Chromium remnants from attempting to launch at startup. Open Safari and choose "Preferences" from the drop-down that appears when you click on "Safari" along the top of the screen. Click the "Privacy" tab that appears. Choose the option "Remove All Website Data" and confirm the choice by selecting "Remove Now." Instead of removing all data, you can instead click on the small "Details" button that's right below the "Remove All Website Data" button. From there, you can scroll through a list and remove only the data you select. If you also use other browsers, such as Firefox or Google Chrome, use an online tutorial to help you clear your website data from them as well.
Stop "Chromium" and "Chromium Helper" via the Activity Monitor. Delete Chromium materials you find in "Library" folders. Trash the Chromium application in the Applications menu. Adjust your System Preferences to prevent automatic startups. Remove website data from Safari and any other browsers.