Spark broke off flush to hole at bottom threads nothing left to grab onto for complete removal???
I have to assume “Spark” is the spark plug. Depending on what is left, it needs to be drilled and a “Easy-Out” or similar used. If that doesn’t work, you may have to removed the cylinder head and have a machine shop remove the stub and repair the threads.
If a mechanic is doing this, they should already know this. If it is you or a friend with limited mechanical knowledge, it is time to call in a professional before you really screw this up. It will be cheaper in the long run.