I am a Property Inspector. Obviously, I carry out property inspections and then send a letter to the property owner informing them of my inspection results.

There are other property inspectors where I work, including my immediate superiors, whose job is to conduct subsequent inspections of the same property. However when they send their inspection result letter to the property owner they sign the letters with my signature block. I am never informed of the fact that they have written a letter and used my signature block.

Is this practice legal ?
We can help people with grammar, not legal advice. Why don't you offer to make them a Word template with their own signature block so they are are less tempted to use yours?

(I assume they are not signing your name, but their own.)
Many years ago I worked as an administrative assistant.  On several occasions I had to sign for my boss.  I was taught that it was okay to do this as long as you signed your own name and added "for _________" (persons name you were signing for).  Is this still considered an okay business practice?
Students: We have free audio pronunciation exercises.
I'd say it's up to each office to come up with their policies on this. I have done the same thing - I sign my OWN name, and then handwrote the word "for" before the signature block of the person whose name the letter was going out over.