First of all, your ISP’s statement that they did not change anything in their procedures for spam or junk emails is… Well… I think I should not comment on that because… Well… It is… Pathetic?
Fighting spam is hard. I have been doing it for 20 years for my personal accounts and it is a constant cat and mouse game. So, I stopped doing it on my own eventually a few months ago. These days, I am using a web host that is working with https://www.spamexperts.com and it is just great. Spam is not being delivered (not even to my spam folder, and once a day I receive a report with further options if something went wrong).
From their website:
Eliminate spam email before it reaches your network with our proprietary, predictive, self-learning technology. Learning from millions of emails processed daily, the proprietary incoming email filtering system has a nearly 100% filtering accuracy rate.
But that is no option for you as a customer of an ISP because it would be the ISP’s decision to use something like that. And it is not free. But it is not expensive either. My web host bills me 18 Euros per year per domain. No need for apps, rules, configurations, whatever…
You might want to look into solutions like SaneBox or SpamSieve. More on that:
So, what has happened? There is a new spammer in town who got hold of your address and the spammer is doing his job: sending spam mails. 
Playing with different encodings is something spammers have been doing for a long time. Bad spam filters are not able to keep up with stuff like that.
And Cox apparently does not care or is not able to acknowledge what is going on. It should be easy for an ISP to monitor its systems and to discover spam like the one you are receiving. But ISPs do not work very hard to achieve that.
Then again, there may be options for you as a Cox customer you can activate, but probably not.