Is it sportsmanlike to waste opponents' time by giving check at the end of the game? In Blitz games, with very little time left for both players, is it acceptable to keep on checking the opponent and waste their time and win the game on time? Is that true sportsmanship?
That only works in blitz time controls with no increment. If you accept games with no increment you are basically agreeing that flagging is part of the game and sportsmanlike. If you personally find it unsporting then always play with an increment and decline challenges with no increment.