The whole one end user / ip address / computer thing is to protect casinos and poker rooms from the endless streams of multiple ID scammers that go around.
Depending on the type of internet connection you have - you probably don't even have permanent IP addr anyway - so the issue really comes about when multiple accounts login from the same PC - which is easily detectable thanks to the footbprints the software leaves.
In this case the whole agruement seems bogus anyway - and it seems to be a frequent cop-out at William Hill (based on the complaints here anyway).
Are you able to take this to IBAS - as I think you'd probably get a favourable ruling - I don't think that an independant arbitrator would agree that using a "friends" computer is materially abusing their policy.
However, even if you win - you're done with William Hill - as I'm sure that they won't have you back ....
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However - how many times have we heard tales similar to this now?
And while the player appears to be knowledgeable they never seems to know about the single account rules ?
And it's always a "friends" computer that must be the issue ?
We've all read the posts and indignant diatribes from the scammers - claiming that they didn't do it - it wasn't them .. etc.
no wonder the helpdesks don'rt believe anyone anymore ...
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Good luck with it mate - and let us know how it turns out.