釘を刺す（くぎを さす） is to warn someone and make something perfectly clear.

「釘を刺す」（くぎを さす）is to warn someone and make sure of something beforehand so that a problem may not happen later.

It literally means to fasten something with a nail. However, 「釘を打つ」（くぎを うつ）is commonly used to mean to fasten something with a nail actually.