force back vs repulse

force back is not a noun while repulse is a noun.

Word	Noun	Adjective	Verb	Adverb
force back	No	No	Yes	No
repulse	Yes	No	Yes	No

As verbs, force back and repulse are synonyms defined as:

Other synonyms of force back include beat back, drive, push back, repel.

force back (noun)	repulse (noun)
	an instance of driving away or warding off

force back (verb)	repulse (verb)
cause to move back by force or influence	force or drive back
	cause to move back by force or influence
	be repellent to; cause aversion in