remove_if()とerase()を組み合わせれば出来る

#include <iostream>
#include <string>
#include <algorithm>
#include <functional>

int main()
{
std::string s = "aaaabbbaaaabbbaaaaa";
std::cout << s << std::endl;

s.erase(std::remove_if(s.begin(), s.end(), std::bind2nd(std::equal_to<char>(), 'b')), s.end());
std::cout << s << std::endl;
}