The problem is that if I delete the account, this guy will notice the account deletion and would want to spam again
He's not a bot, but a member of an organization called "AUFA" (Anti Ugly Forum Action) who is known to spam many boards. He previously spammed the French support forum before he came to spam here.
We are currently waiting for the technicians' reply about a possible abuse report against these AUFA guys, so we might need to give info to collect the proofs against them.
I can always change the username for "spammer" and wait three months before deletion.
EDIT : I sent a PM to a Jurid on the French support to explain this situation and I'm waiting for her reply.