How do I cancel the renewal for Classic Invision ($200)?

Reach out to the account team. 🙂

https://invisioncommunity.com/contact-us

A question, if I canceled renew of classic license, can my forum still operational, till I decide to renew?

Your site itself will remain operable if inactive, however you cannot upgrade or use any IPS related services such as support, spam service etc. Of course after a 2 year period your license would be permanently expired, so you would then not be able to renew