Possible to evict tenant who is illegally subletting without lease signature?

[John WIlson]

A tenant of mine has been living in a rent controlled unit since the '80s & pays $1000/mo. for a 3 bedroom (market is $3,800). It has come to my attention that a new sublessee has moved in a few days ago. The original tenants do not live there, however they still pay us monthly. I am almost positive they are making bank from this sublet situation. We have the original lease, which prohibits subleasing, but we are missing the last page which has a signature. We are willing to do anything within the confines of the law to bring this unit to market again. Do we have any recourse? Thank you.