1 what material should I use to block entry through the pipes?

Gap-filling foam? Should slow them down enough, and can be semi-easily
removed later when you can fit something more permanent (proper grommetted
holes).

2 when should I tackle the removal of the nest?

Yesterday!

3 when should I block the entrance off?

Now.

Nippon powder if you can get close enough, or the spray foam wasp killer
(both available from the average hardware store, garden centre, pound
shop). Aim to cover the landing points where the wasps enter, and coat the
whole nest in powder or foam.

Al.