A common exception to the Fourth Amendment's protection against warrantless searches is consent. When you consent to a search, law enforcement can search as much as you consented to, whether that's probing your person, home, car, or stuff. The prosecution can typically use whatever the officers find as evidence against you in a trial. But, if you grant consent, then change your mind about the search once it begins, you may be able to stop the search—under certain conditions. (Schneckloth v. Bustamonte,412 U.S. 218 (1973); U.S. v. Oyortiz, 669 F.3d 439 (4th Cir. 2012).)
Typically, if you withdraw your consent to a search that's underway, the officer must stop searching immediately. You must be very clear that you are withdrawing your consent. You can indicate intent to withdraw by making a clear, unambiguous statement, or by taking some clear, physical action. (U.S. v. Ross, 263 F. 3d 844 (8th Cir. 2001).)
While there are no set words you must use, "I am withdrawing my consent to this search" will probably do the trick. Something like "That's enough; I want you to stop" may also work. On the other hand, merely expressing disapproval of the search, or that it is inconveniencing you or taking too long, probably isn't clear enough to show your intent for the search to stop. (U.S. v. Gray, 369 F.3d 1024 (8th Cir. 2004).)
It's sometimes possible to withdraw consent by physical action. In some states, pushing an officer's hands away from an item he is searching is enough. For instance, someone might push the officer's hands away from her pockets or box of cigarettes. (Take note that using any degree of force in interacting with an officer can lead to big trouble—see, for example, this article on resisting arrest.) A similar way to withdraw consent might be attempting to retrieve the item being searched. (Lowery v. State, 894 So. 2d 1032 (Fla. Dist. Ct. App. 2005).) Be aware, however, that not all courts may consider such actions an adequate indication of intent to withdraw consent. Also be aware that getting "physical" with an officer can lead to arrest, and even use of force by the officer. In general, it's best to use one's words.
In addition to being free to withdraw consent for a search, you can also limit a search to particular items. But you must generally make a statement that clearly and unambiguously indicates your intent to limit the search. If you make clear that you are consenting to a search for drugs only, for example, the officer probably cannot start reading papers (clearly not drugs) he finds during the search. (U.S. v. Dichiarinte, 445 F.2d 126 (7th Cir. 1971).)
There are situations where you probably cannot withdraw your consent to a search. Among them are the following:
Not surprisingly, it's typically too late to withdraw your consent after an officer has found something incriminating. Whatever the officer finds legally before you withdraw that consent can be used against you.
Once you've started the process of being screened at airport security, even if the screeners haven't yet found anything incriminating, you typically cannot withdraw consent. (U.S. v. Herzbrun, 723 F. 2d 773 (11th Cir. 1984).)
As in the airport-screening situation, you generally cannot withdraw your consent to a search once you've begun the process of being searched before a prison visit, where visitors are informed beforehand that they will be searched. (U.S v. Spriggs, 827 F. Supp. 372 (1993).)
PLAYING IT SAFE
Oftentimes, police officers don't tell citizens that there's no obligation to consent to a search (the law doesn't require them to). Though the circumstances may be intimidating, one need not volunteer to be searched or agree to an officer's request for consent to search. If the officers have a lawful justification to search someone, they don't need consent. They often ask for consent when they don't have a justification at the beginning of a search.
Many who have politely, but firmly, stood their ground and made clear that they do not consent to being searched both before and during a search have protected their Fourth Amendment rights. Being clear that one does not consent to a search may prevent the search or prevent the prosecution from offering into evidence any incriminating evidence the police find. (See What is the exclusionary rule?)
Even when it comes to search-and-seizure cases, the law may vary somewhat from one place to another. For example, even though the U.S. Constitution sets the "floor" for basic freedoms, one state's constitution may vary from another's. Only a criminal defense attorney with experience in your state can adequately apprise you of the law that applies to your situation.