Spam back, use anticipation and play a strategic game instead of a reactive one. Let me give you an example of an interaction(that, by the way, is intensely difficult to beat and one of the best spam tactics please dear lord don't copy this if you play against me)
Mitsurugi: 1a, 1a, bb, a, 1a, 1a, bb, a, 66bb, 1a (you notice they don't block at all)
You: Dead and also very pissed
Round 2
Mitsurugi: same
You: Start with spammed fastest attack, still get hit :-(, Hold Guard(get hit), Hold Guard(get hit), Hold Guard(notice you blocked the hit, immediately begin a while-crouched low kick, bb, bb, bb, while-crouched low kick, bb, DO NOT STOP ATTACKING WITH YOUR BEST MIXUP), if you are knocked down or fall to the mixup, you have two options now
1) try to guard, sidestep, parry(difficult)
2) spam your fastest attacks and try to gain advantage (easy)
Round 3
Mitsurugi: slightly different strategy, more aa and 1a, less bb
You: Counteract strategically with guarding low to gain advantage, again keep attacking with your best mixup
Round 4
Mitsurugi: slightly different strategy, mostly bb
You: Finding it difficult to gain advantage by using frame advantage you decide to parry the predictable spam and lo and behold it is easier than you expected because you limited the spammer's options. This is the case where it is easier to fight spam...when you get inside their head. Unfortunately pro gamers are suited to the task and will outplay you strategically...but you're probably not at that level so use this as a learning experience and learn their mixup, their strategy and timing, and get better by learning from your opponent.
Sure, you're not going to win without spamming and you're only learning strategy but that is valid unless you're above a certain level. In fact, mixup and pressure is one of the fundamentals of soul calibur and one you need to learn. Once you know it and also know your best strategic attacks etc you probably will be even more upset that soul calibur 4 was designed to be such a shitty game online. But hey, soul calibur 5 may not take 3 years to come out you never know.
I will never spam online to victory, I was able to lose to a level 50 yesterday because I was more interested in his attack patterns and toying with him and defending(futilely online) than spamming and showing him the online bullshit you see in most online ranked games. In fact, if I get 10+ parries off I consider it a win for me because I've compensated and shown myself a high level of play under the most adverse circumstances. Sometimes, it's seeing yourself overcome obstacles and brace the torrent of online spam that makes you have fun. If you're playing online against spammers, you need to remember it's not "all about the W" it's about having fun and learning from your mistakes and the strength of others. If it is "all about the W" feel free to copy the bullshit about fighting spam and pervert it against non-spammers but I encourage you to use the first round as a "feeler" and see if your opponent really is a dickwad who likes to ruin online interaction with a strategic spam. If they are...fight with strategy and spam...but if they aren't relish your opponent and the interactive game that is soul calibur (pseudo-offline) and you might even want to send them a gg and start a friends list with those folk because they are few and even fewer playing in ranked matches.