Yes, some symptoms can be treated.
The positive symptoms of schizophrenia (hallucinations, posturing, etc) can be treated with drugs, though the negative symptoms (difficulty taking care of oneself) cannot.
Other mental disorders like depression can be treated with therapy, sun exposure, or drugs, depending on the type.
I'm not sure about other forms of treatment for specific disorders other than drug treatments, but basically any symptoms that have neurotransmitter problem could potentially have a drug available to treat it. Since I don't know too much about psychology, and there are over 400 mental disorders, I can't really tell you the specifics.