The only way is to call customer service as you need a working sim to do it any other way.
When you call to report the phone stolen, customer service will sort out a replacement sim for you or activate a spare blank one if you have one.
https://community.o2.co.uk/t5/Other-Products-and-Services/Lost-or-Stolen-O2-Device-How-to-Report-it/...
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion. This is not Customer Service so we are unable to help with account specific issues.
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