The protesters, one of them dressed as Cap'n Birds Eye, handed out leaflets headed: "Unilever - the Grimsby Reaper".
James Hill, chairman of Unilever Ice Cream and Frozen Food, spoke at the conference about the issues facing food companies operating in the UK.
He told the convention that Unilever had no option but to close its 60-year-old Grimsby plant. "The bottom line was that our factory was at a competitive disadvantage to our rivals with bigger scale and lower cost."
But in its leaflet, the GMB said: "The plant is profitable - it made £20m last year - and highly productive, contributing towards Unilever's global profit of £2bn in 2003.
"Its closure will decimate a community."
And the union added: "What type of behaviour is this from a global company that protrays itself as a beacon of corporate social responsibility?"