expression, godan verb (-ru)
be given up; be discontinued; be called off
Used when a plan, event, or activity is cancelled or abandoned, often by external decision or circumstances.
雨で運動会はやめになった。
The sports day was called off due to rain.
その計画は結局やめになった。
That plan ended up being abandoned.
止める is the transitive verb meaning 'to stop (something)' or 'to quit', while 止めになる is an intransitive expression meaning 'to be stopped' or 'to be called off'.
Formed from the continuative form 止め (yame) of the verb 止める (yameru, 'to stop') + になる (ni naru, 'to become'), literally 'to become stopped'.