help how to disable buttons

This is my code guys. I need to disable button 1, 2 and 3. Example when I push button 1, I can do it only once. I also need a reset function so that I can use again button1, button 2 and button 3.

// include the library code:
#include <LiquidCrystal.h>

// initialize the library with the numbers of the interface pins
LiquidCrystal lcd(8, 7, 6, 5, 4, 3);
int count = 60;
int start = 12;
int reset = A5;
int startTime;
int buttonStart;
int buttonStop;
int buttonReset;
int button1Pin = 9;
int button2Pin = 10;
int button3Pin = 11;
int button1 =0;
int button2 = 0;
int button3 = 0;
int buzz1;
int buzz2;
int buzz3;
int answer=0;

void setup() {

pinMode(start, INPUT);
pinMode(reset, INPUT);
// pinMode(led1, OUTPUT);
//pinMode(led2, OUTPUT);
//pinMode(led3, OUTPUT);
pinMode(button1Pin, INPUT);
pinMode(button2Pin, INPUT);
pinMode(button3Pin, INPUT);
pinMode(2, OUTPUT);

lcd.begin(16,2);
Serial.begin(9600); // setup serial
}

void loop() {

buttonStart = digitalRead(start);
buttonReset = digitalRead(reset);

button1 = digitalRead(button1Pin);
button2 = digitalRead(button2Pin);
button3 = digitalRead(button3Pin);

if (buttonStart==HIGH){
startTime=HIGH;
answer=0;
}

if (startTime==HIGH)
count–;

if (buttonReset==HIGH)
{count=60;
answer=0;
}

if (count==0)
startTime=LOW;

if(answer==LOW){
if (button1==HIGH){
digitalWrite(2,HIGH);
delay(100);
digitalWrite(2,LOW);
startTime=LOW;
answer=HIGH;

}
if (button2==HIGH){
digitalWrite(2,HIGH);
delay(100);
digitalWrite(2,LOW);
startTime=LOW;
answer=HIGH;
}
if (button3==HIGH){
digitalWrite(2,HIGH);
delay(100);
digitalWrite(2,LOW);
startTime=LOW;
answer=HIGH;
}
}

lcd.print("TIME ");
lcd.print(count);
Serial.println(answer);

delay(900);
lcd.clear();
}

Don't think in terms of disabling the button. Think in terms of deciding how to react to your button. Set up a Boolean variable for each button. When you use that button set the variable to true. When you see the button is pressed check that variable. If it is false then react but if it is true do nothing about it. To start using that button again set the variable back to false.

make disabled flag variables you can use to keep track of and control which buttons become inactive

...

bool disabled1 = false;    // initialize to not disabled for button1

...

if ( !disabled1 && (button1==HIGH)){   // only respond to button1 action HIGH when not disabled1
   digitalWrite(2,HIGH);

...

Then just figure out where you want to set or unset the disable flags. Like your reset task would be accomplished just by setting them all to false (i.e. not disabled). It might be easier to read and follow the code if you make them enable variables and reverse the logic around. So they keep track of which buttons are enabled instead. Which way is easier depends on which angle you are looking at the problem from. if( enabled1 && ... tends to read better than if( !disabled1 && ... does. Likewise when you say if( !enabled1 && ... then maybe if( disabled1 && ... would be clearer.