Zonal geraniums bloom in a cluster of individual flowers held at the top of a bloom stem. Sometimes the flowers in a single open a bit sporadically so it is very possible to have some spent flowers in amongst others that still look good or are even not yet fully open (buds).

It is your choice as to whether you leave that flowerhead in place so that ALL flowers open or deadhead it after some/most of the flowers have gone over but some buds may still remain. Since zonals are so prolific in their flowering in summer, I remove any flowerheads that are even starting to look ratty. And yes, you remove the complete flower stem from where it emerges.

The flower stem will snap off crisply if you hold it near the base and bend it back. Don't cut it off because that will leave a stub.