unwanted vs undesirable

unwanted is not a noun while undesirable is a noun.

Word	Noun	Adjective	Verb	Adverb
unwanted	No	Yes	No	No
undesirable	Yes	Yes	No	No

As adjectives, unwanted and undesirable are synonyms defined as:

unwanted (noun)	undesirable (noun)
	one whose presence is undesirable

unwanted (adjective)	undesirable (adjective)
not wanted; not needed	not wanted
not wanted	not worthy of being chosen (especially as a spouse)