This example shows how to ignore one or more files during compilation.
If you run npx hardhat compile in this project, you'll get an error because Bar.sol has a syntax error. If for any reason you want to ignore that file temporarily, you can override the TASK_COMPILE_SOLIDITY_GET_SOURCE_PATHS subtask. Go to the hardhat.config.js file and uncomment the code that does this to see how that can be done.
Since a Hardhat config is just code, you can take any approach you want to do this. For example, you could add a new entry in config.paths, like whitelist, to specify the subset of files you want to compile, and then use config.paths.whitelist in the overriden task to include only those files.