合う => do something together N3
禁止形 => Negative imperative form N4
~こと請け合い => I guarantee ... N1
Happens next
[禁止「きんし」] => Don't do N4
Can, Should, Must
[合「あ」]う => To do A together/mutually N4
Secondary Verb
場合(は) => in the event of; in the case that ~ N4
~ににあわず (に似合わず ) => ~Different from N1
Evaluate
~をきんじえない(~を禁じ得ない) => can't help but; can't refrain from ~ N1
を[禁「きん」]じ[得「え」]ない => Can't help but do A, can't hold back from A (emotional response to something) N1
Will, Decision, Doing