The answer to this varies by state, but there are courts in the United States that have ruled that a person who is engaged in a sexual encounter with another person must stop if the second person withdraws consent. If the one partner withdraws consent and the other does not stop, these cases have held, the partner who does not stop may be charged with rape.
The links below refer to a case in Maryland and one in California. In both cases, the courts found that a person may withdraw consent and the partner must then stop. However, North Carolina law, for instance, has held just the opposite -- that a person who has consented to start a sexual encounter cannot withdraw consent.
So, the law varies from place to place on this issue.