But again as i said in a different older thread.
If it was definitely a self exclusion then that has a time-frame as above post mentioned. maximum time-frame is 5 years you can do them for. Some casinos have different minimums and maximums though.
Or if you just asked them to close account and didn't mention the words Self Exclusion to them, then that is a totally different thing. And if you said Permanently when you asked them to close it.
That is then down to the Customer Service Agent to either add a note to that account noting that you never want it reopened ever. And the fact that they didn't honour your statement if you did say that, then they are partially to blame but if you do not have that evidence of that chat log from 2015 which i presume you don't, then they might have deleted their copy as well.
And because they did reopen it, I presume they didn't honour your request if what you said was true and you said Permanently.
Best bet is you ask them to send you that chat transcript from 2015, If they refuse. Then you might have a case but otherwise if they do send it to you and you didn't word it how you worded it in your OP post, then you will not have a case.
Just remember 2015 is a long time you might not exactly remember your wording you used. I don't remember all the chats i have with casino live chat agents.
But i hope you can get that chat transcript and post it here, just remember to block your name from it and your personal info.
So keep us updated and good luck.