Where are you from? The laws on things like this differ depending on the country you live in. In the UK, it is against the law to place a sex offender in a property if all the residents are not comfortable with it, especially if there are children in the property. If all the residents complain, the landlord (if it's a private landlord) can legally evict this person. As Lark said, if there are any parole violations he can go back into prison. The parole officer could come once a week, month or even once every day in severe cases, so try to catch them on a visit.
Also, if the drug addict is still using the drugs and you think some can be found in his property, call the police. Explain how you fear for your safety, they might be able to get a search warrant. But there's not much you can do about the addict, unfortunately. He will probably recieve a warning, a stint in rehab or a very light punishment.
However, it is tricky because if the paedophile or drug user isn't actually causing any problems, the police could just state that they are reformed criminals and should not be discriminated upon because of their past.
In the meantime, for peace of mind you could cast a protection spell.