I have a reseller account and last week 1 of my client have sent out spam email and my reseller account ip was null-routed. Now my the other client site also cannot access because of this problem.

I want to know in this case, does the hosting provider should null-routed the ip or not because this is just 1 of site illegal the rules and now all of the other site also cannot access. Why don't just suspended the account who sent spam ?:(

If your a customer made SPAM when your hosting provider must suspend your customer's account only. Ours and most other hosting providers do this. And you must contact your customer and your hosting provider about the subject. If it maid SPAM really when you remove it's account on the server.

it will entirely depend on who you get your reseller account from and what their policies are.

have you tried discussing it with them to get the ip unblocked in exchange for terminating the account that sent the spam ?

have you check that it really is blocked by the host rather than the IP appearing in spam blocklists ?