If police don't have probable cause for a search, they might ask you to consent to a search—it's a common and legal police tactic, assuming police don't use coercive tactics. Once you've consented, can you change your mind? In many instances, you can withdraw your consent or limit the scope or extent of your consent. But your withdrawal or limitations must be clear, and you can't withdraw your consent once evidence has already been found.
Many people don't realize that they don't have to consent to a search, and the badge and gun certainly don't make it easy to say no. When you consent to a search, law enforcement can search as much as you consented to, whether that's searching your person, home, car, or stuff. The prosecution can typically use whatever the officers find as evidence against you in a trial. If you've already consented, changing your mind is an option if you act clearly and quickly. (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 stating the search is an inconvenience or taking too long probably won't be 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 or blocking an officer's hands can be enough to indicate you're withdrawing consent. 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—and could be considered resisting arrest.) A similar way to withdraw consent might be attempting to retrieve the item being searched. Completely blocking an officer's path or locking a door might also suffice.
Be aware, however, that not all courts may consider such actions an adequate indication of intent to withdraw consent. Also, 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. (Lowery v. State, 894 So.2d 1032 (Fla. Dist. Ct. App. 2005).)
In addition to being free to withdraw consent for a search, you can also limit a search to particular items or places. 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) found during the search. Or you can indicate that you are consenting only to a search of your house and not to the garage or shed. (U.S. v. Dichiarinte, 445 F.2d 126 (7th Cir. 1971).)
There are situations where you probably cannot withdraw your consent to a search.
Evidence found. 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.
Airport screening. 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).)
Prison visitation. 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).)
Even when it comes to search-and-seizure cases, the law may vary somewhat from one state to another. For example, even though the U.S. Constitution sets the "floor" for basic freedoms, one state's constitution may provide broader protections. A criminal defense attorney can help you understand the law and how it applies to your situation.