In my extensive experience fighting spam on Super User, I've noticed that spammers often use many different domains to evade detection. In several cases, I've found that the domains often resolve to one or a few IP addresses, which are used exclusively by a particular spammer or spam campaign. An example is 184.108.40.206.
To defeat this evasion technique, I suggest adding the ability to blacklist domains which resolve to certain IP addresses which are found to be hosting spam. This blacklist would be enforced against users with less than 10 reputation (the reputation level at which new user restrictions are lifted), where any posts which link to a domain that resolves to a blacklisted IP would be rejected. (Users with at least 10 reputation would still be allowed to make edits that add links to domains that are blacklisted in this manner.)
As I envision it, the blacklist should be manually populated, with some degree of automated assistance. Ideally, the system would examine posts that are flagged as spam, resolve linked domains to their IP addresses, and report to Stack Exchange developers the destination IPs that are most commonly spammed. The community team (or perhaps ♦moderators) can then check if the IP is used exclusively for spam using online tools such as VirusTotal to perform reverse DNS lookups (to avoid blocking legitimate shared hosts). Because IPs can and do change with time, each IP address on the blacklist will need to be reevaluated periodically to avoid blocking legitimate websites which may later use this IP address. My list of known spammers would serve as a good starting point for populating the list.