It is better to refuse doing that. You can say that you are too busy to help them.
My ex-boss had the same situation like you. He left our company, but they kept calling or emailing him for a long time. At one emergency time, he helped the company to bring the system up by working continually 24 hours because they promised to pay him, but it took the company 4 months to give him a check due to all kinds of political reasons(he is not employee, he ...........). He ended up with angry. The relationship now is broken.
So, do not let them depend on your help. It is not your responsible to help. It is also good for you to keep the existing good relationship this way.
Hope this helps.