If Procter & Gamble were sponsoring a 2018 advertisement for avoiding activists, it might include an in-house remedy for removing corporate stains. Investors seeking new strategies, more dividends, executive changes or board seats aren't going away. If anything, the siege of P.&G. by Nelson Peltz, whose Trian Partners gained a director's seat after a 10-month campaign, will embolden others to take up the cause. Resistance isn't futile, but it needs to be more carefully considered.