ip addresses are usually dynamic, one of the spammers at some point had that ip address in the past, everytime you restart the router it "rolls the dice" one of thoose rolls had this flagged ip address...
fun fact, why are ip's dynamic? every isp get (or buy, or rent or are attributed, i don't know how exactly they got hands on a "deck of ip's") a number of ips ranges. for example if the isp got 1000 ip's reserved for them. because not everyone is online at same time they can have 2000 costumers, it's the same thing with airplane tickets.. a company sells a percentage higher of tickets than the seats available at a plane because there are always costumers that don't show up at last hours...
(numbers are examples. and i am talking about ip v4)
and if you ask why do they ban ip's since they are always changing.. keep in mind, if you have a server or are on a company, you can have a fixed ip, so it kinda works...