Oh, right. If you try anything funny with Mayu, I'll break your spine.
Literal
Ah, right. Mayu [to-に] weird thing do-[if-たら], spine [obj-を] break.
A blunt, short threat — plain-form 折る as simple future/declarative 'I will break.' 変なこと ('weird thing') is a common euphemism for inappropriate or sexual behavior. Particle に on the target of the 変なこと marks the person the action is directed at. ~たら is the most common everyday 'if/when' conditional, flavored slightly more scenario-specific than ~ば.