The 4th Amendment to the United States Constitution guarantees that all citizen of the United States have the right to be free of unreasonable searches and seizures. If law enforcement violates your rights, none of the evidence obtained as a result of the illegal search and seizure can be used against you, and often the case against you has to be dismissed.
However, if you consent to a search, law enforcement can search without a warrant, and they will often search in more areas in and more places than you originally allowed.
If law enforcement asks to search you, your home, or your car, tell them that if they do so it is without your consent. Look closely at anything that you sign and demand a copy of it immediately to be sure you aren't consenting to be searched in writing. The reality is that they may search anyway, but at least your consent can't later be used to justify the search.
Always be respectful of law enforcement and don't physically stop them from searching, but politely make it clear that your consent was not given for the search.