AFAIK IANA and the RIR's cannot enforce use of IP space assignments on any
network.

<chuckle>  While route hijacking isn't necessarily an ARIN issue, I will note that several US law enforcement agencies (FBI & NCIS Cybercrime units) are quite interested in such events and do investigate them looking for criminal activity.   

(See https://pc.nanog.org/static/published/meetings/NANOG77/2108/20191028_Elverson_Your_As_Is_v1.pdf for details.) 

FYI,

/John