Can i can return 404 if file not exists?

Hello can i return 404 or 403 if anyone try to open /?a=b AND 1=1

My vhost is:

server {
listen 80;
listen [::]:80;
server_name mysite.com www.mysite.com;
root /var/www/mysite.com/public_html;
index index.php;
rewrite ^ https://$server_name$request_uri? permanent;
}

server {
listen 443 ssl;
listen [::]:443 ssl;
root /var/www/mysite.com/public_html;
index index.php index.html index.htm index.nginx-debian.html;
server_name mysite.com www.mysite.com;
error_page 403 https://mysite.com/dead.html;

location = /dead.html {
allow all;
}

location / {
try_files $uri $uri/ =404;
}

location ~ \.php$ {
try_files $uri =404;
fastcgi_pass 127.0.0.1:9000;
fastcgi_index index.php;
fastcgi_read_timeout 240;					 
fastcgi_param   SCRIPT_FILENAME    $document_root$fastcgi_script_name;
include fastcgi_params;
}

location ~ /\. {
deny all;
}

location ~ /\.ht {
access_log        off; 
log_not_found     off; 
deny              all; 
}
}