You can use 名花有主 to describe a third person 'being taken' in a romantic sense. For example: "他已经是名花有主" (he is already taken)
Since it sounds arrogant to call oneself "名花" (quite a catch), "名花有主" is only appropriate when describing a third person.
Also "名花" is usually a description of a lady. Using it to describe a man is less common, therefore someone would say "名草有主" instead
Or you can use "心有所屬" to declare you are "taken"
"我已经心有所屬" = (my heart had already settled on someone)
But 心有所屬 is quite a literary phrase, rarely used colloquially.
It sounds most natural in native Chinese speech