Not oil. Oil is an easily substitutable commodity. Lots of sellers, lots of buyers, almost all of them trading worldwide across the ocean blue.
You just whistle up a tanker bringing oil from somewhere else.
The guy you are boycotting will just sell to someone else. Net-net its a wash.
What you probably mean is natural gas. That is more like hooking up a utility service. Even so, for the most part Nat Gas is substitutable without too much effort. Most is used for electricity generation and there are several ways of producing electricity, some of which Germany has or was planning to mothball, like Nuke and Coal plants.