Is a lease violation legal without the landlords handwritten signiture?

Tell us your state and I'll look it up but I'm 99% sure that it's legal.

What do you mean by "lease violation"?

Did the LL given you written notice of something pertaining to the lease, such as a notice of cancellation, notice to quit, or whatever? If the LL was clear about what was communicated to you, I doubt that the formality of a signature is that important.

A lease violation is dependent on whether in fact a violation occurred. Not whether it was signed off on.

Without knowing the background here and the real issue its really hard to give you a decent answer. Also any question on law needs to include your general locale as laws vary by area.