I'll probably be reitterating what everyone else has said, but an Nth opinon on the same view probably couldn't hurt.
How are overhead attacks blocked?
Block while standing. If you're confident in your timing and spacing, you can use 2b to attack back (beating many different types of overheads and jump ins) or use 5d to shield (taking care of anything else 2b can't), but the safest option by far is to block while standing.
Example: Sion j. C
You can block this while standing.
You can beat this out with 2b, sometimes getting a counterhit and can do a relaunch combo.
You can beat this out with 5d~2a counterhit combo.
What to do against spammers?
Ah yes, being able to defeat people who mash aaaaaaa or EXs is a sign of maturity for a Kouma player. There are lots of ways to do this and signficantly different answers to many different situations, but they all have the same goal. On offense, the greatest weakness to reversal attacking is that it's (often) less safe than blocking. You will need to be able to punish this difference in safety by either closing the gap in your blockstring or by baiting and punishing the reversal attempt. On defense, this is more difficult, but you need to block and wait for an opening to stick an attack through.
Example[Offensive]: Nanaya 2a
Say you're performing your block string 2b 5b 5c 2c, one way to bait and beat the reversal 2a is to create an intentional gap between 5c and 2c, then attack with 2c BEFORE Nanaya's 2a can completely come out.
Example[Defensive]: Nanaya blockstring
Nanaya does his block string 2b 2c 5c 5a(whiff). It's possible that when Nanaya whiffs his 5a to try and perform a reversal 214c. If Nanaya dashes at you after doing a whiff 5a, the 214c will it cleanly
Though, I'm not a fan of 214c, because it only works in those kinds of situations, but you get the idea right?
This is just out of curiosity, but how do you perform Kouma's arc drive?
As said before, 41236c. It's not that good though, but it has some pretty big AWESOME factor.
What are some good poking maneuvers in a spamming situation (when your getting spammed)?
As stated before, if you're getting attacked by an opponent, Kouma's best option is to block and wait for an opening. There is no such thing as an unlimited blockstring in Melty Blood; all blockstrings have framegaps than an opponent can punish if he is looking for it. Kouma has a nearly unlimited number of options he can perform to escape various situational pressure, but it's essential to Kouma's game plan to block and find an opening instead of recklessly trying to reversal his opponents because his reversals are not as omnipotent as other characters.
Against ground:
22c
214c
5/2d
2a
2/5b
Against air:
2b
5d
623a/b/c
Another (safe) option outside of waiting for framegaps is to use the bunker cancel to reversal your opponent. This takes a lot of practice, so check out the thread itself if you're interested.
http://www.meltybread.com/forums/index.php?topic=6.0