Defense against Activists wastes Investor's Money

Why can a company like P&G not just say NO to an 'Activist' speculator who pesters it? If Peltz wants he should rustle up enough votes to hold a shareholder meeting. Apart from that there should be protection against manipulating the shareholder register by stake building, only allow longterm holders to vote. Now THAT would be proper defense against unwanted approaches, it it would cost NOTHING, to the detriment of corporate finance 'advisers'.
(21-Sept-2017)
https://www.valuewalk.com/2017/09/pg-spending-100-million-fend-off-nelson-peltz/