Can someone else use a node I cancelled

Not while an Allowed record remains.

After you cancel a node with the provider, that IP may go back into their pool and another Merca user may buy it. On Merca, used IPs stay in the global account records for good.

If you still have an Allowed record, their launch of a different account on the same platform is blocked, and they cannot Allow a new record. After you Delete the Allowed record, other accounts can launch; their own Blocked row still stops that login. The row stays as Deleted so later users can see this IP was used. How that check works is in How login protection works.

What contamination means is in What is node contamination.