Can you physically stop someone from commiting suicide? Once you write suicide as the cause of death, the Death Note possesses victim and they commit suicide e.g. Naiomi Misora.

What would happen if you were to physically prevent that person from killing themselves?

For example, if Light were to physically restrain Naiomi before she hung herself, would the Death Note default to heart attack since the victim is incapable of fulfilling it?
Second paragraph from "How to Use: LV"


  In the occasion where the cause of death is possible but the situation is not, only the cause of death will take effect for that victim. If both the cause and the situation are impossible, that victim will die of heart attack. 


So yes, the victim should have a heart attack if they are unable to kill themselves at the specified time.