OK, I'm really going to depress you - pores are not doors, they do not open and close. They are the shape that they are and you can't 'close' them.
So, dose of reality out of the way, what can you do? AHAs (glycolic, mandelic and lactic acids) + BHAs (salicylic acid) will exfoliate and make the skin around them look better so over all they look smaller - this is definitely the case for me since using the salicylic. Basically if you have blackheads use salicylic, otherwise use AHAs.
Other option is retinol - it also exfoliates but also thickens the skin and supports the pores, also making them look smaller. This is usually the special ingredient in night creams as it makes you more sensitive to the sun.
Neither of these are a miracle but they do do something. Helpfully they are also the things that work for spots and anti-ageing so you get both effects as well! If you use either you must use SPF!
Things I've tried are LRP Effaclar, Avene ystheal or Triacneal, Vichy Normaderm (liked them all). Things I want to try - Bravura london, Alpha-H. Favourite mega cheap option - SR Skincare. No frills internet brand (again recommended by someone on here, wish I knew who to say thank-you to) - I use their 10% Salicylic peel weekly and it's brilliant.