If every person in the world spent 30 days without human contact in a sterilized space, would it be possible to eradicate the flu or other diseases?

Nope. Nearly all viruses and bacteria that infect humans have non-human reservoirs, with only a few exceptions. In addition, some viruses are resident in human DNA and can re-emerge from your genome itself with no outside vector and no infection present.

Viruses are with us forever, likely¹. Harmful bacteria might be more capable of being eliminated fully.

1. Absent a whole lot of genetic engineering.