If you don't want BOINC running without opening an using the client, the following will stop and disable BOINC.
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sudo systemctl stop boinc-client
sudo systemctl disable boinc-client
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Don't want it installed on your linux installation any longer?
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sudo apt remove boinc boinc-client boinctui libboinc7
rm /home/pi/.config/lxsession/LXDE-pi/boinc.sh
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