The board of insurance and investment group Australian will shut the door on future takeover bids, with a plan to require at least 20 per cent of members to vote on any plan to demutualise.The board will ask members to change the constitution at a meeting on October 6, The Age reports.The proposal by the board is a tactic to fend off any expected takeover offer from Macquarie Bank.Australian Unity succeeded in merging with rival health insurance group Grand United earlier this year, but only after fending off an alternative offer, and court proceedings, bought by Macquarie.Australian Unity has 180,000 members in its health insurance fund and funds management businesses, and about $2.8 billion in funds under management.