Shirley, You say that the main water is turned off! Lets assume that the water is actually off. the continuing water flow is obviously coming from another source:-
1. backsiphoning from a HWS.
2. a cross conected water supply.
3. residual water from your plumbing.
If the residual water is backsiphoning from your HWS, then if you dont take the correct precautions you may burn out your elements or otherwise damage your HWS. A Licensed Plumber should be contracted.
If there is a cross conection, the integrity of your potable supply is highly questionable. A licensed Plumber should unquestionably be contracted.
If the residual water is from your existing plumbing, a Licensed Plumber would be most able to carry out the repair.
In short you should contact a Licensed Master Plumber, rather than risk the consequences of a cheap job.