There are a lot of really good suggestions here so I'm just going to prioritise what I think is most urgent and what will be the most effective in reducing the spam problem.
Requiring manually whitelisting of all new accounts before they can post. All new users will essentially be shadowbanned until they've been verified by a mod. This will eliminate 99% of bots and spammers before they even start. Spam threads can be trashed on sight before they even become spam megathreads.
This is the only effective way to deal with bots. The other suggestions about captcha and email verification are trivial for botnets and won't even slow them down.
Require at least one merit to become a Junior Member (bots will never rise past Newbie status then and can be nuked once spotted).
Remove signatures completely from lower ranks (at least Juniors). Purchasing a Copper Membership (or higher) could still get you one.
Whichever combination of this is implemented doesn't really matter but until members require at least 1 merit to get a signature there will be spambots by the thousand getting paid in shitcoins/tokens. The same requirement to be immune from the nuke button makes sense as well.
Dedicated sub board mods for most boards that don't have any mods or non-global mods already assigned (Bitcoin Discussion, Beginners & Help, Off Topic etc).
More patrollers/mods who just handle sig spam or farmers.
More admins or demi-admins added to help with account recoveries and other admin duties.
All of these are desperately needed at the moment although implementing the first two suggestions may actually reduce the need.