Well Olimar isn't so scary in my opinion. They usually only deal about 2 to 6 percent before they get knocked off. Just treat them like any other character might treat fox's laser and run through them to punish a spammer. Just watch out for purples. If you find yourself getting hit, don't panic, just do a neutral A to knock them off real fast. Also, be aware that your PK fires will hit the pikmin stuck to you rather than moving outward. So watch out for red ones in particular because they won't even get knocked off by PK fire.
As for Diddy you can catch either of his projectiles just by airdodging through them. I haven't played a good diddy so far, so I dunno if that's a good technique to answer a spammer, but it's something to try.
To answer Mind Your Manners, shielding is slower than spot dodge, especially if you get hit because it'll lock you up for a moment with the impact stun and even move you backwards. Also, the more you use your shield the smaller it gets until some attacks start getting through. Spot dodging's just more reliable.