Here is what I did and my feelings on the matter (open to discussion):
1) I told West in private that his score would be reduced by 3 IMPs for the behavioral lapse -- and that further lapses would lead to larger penalties. He apologized for his outburst to me as soon as I sent the message. I told each of the other players in private that I had penalized West in IMPs and all three were smart enough to realize that it didn't matter to them how many.
I think it is important to give him the bad news in private -- even (especially) in an offline setting. There is no need to make an example of the player on a first offense, especially in an indy where one must put up with many partners... In an offline setting I would take the player away and let him know the penalty I had decided to assess. Too many Director calls in my area seem to stop play at all tables within earshot, as the guilty parties and the penalties are gossip items for the rest of the evening. We need to stop this from being the norm.
I agree with mikestar that in a club setting this would warrant a larger penalty.
2) Law 16A pretty clearly prohibits the spade lead from East at trick four in light of West's outburst at trick one. You can argue that the bidding makes the spade lead obvious but if there is a logical alternative (logical meaning one that might work as well or better) it must be chosen over a spade. The alternative is a heart or a diamond lead (clubs are gone). A heart is clearly a logical alternative, since declarer may be void and has entries to dummy to pitch heart losers on established diamonds. Since the Laws say you resolve any questionable points in favour of the non-offenders, I tried to come up with a logical reason for leading a diamond, but I could not.
So the question is: what happens if East leads the K
♥ instead of the K
♠ at trick four? West will likely encourage and overtake a second heart to cash a spade. Or he may overtake, cash a spade, and play a second heart. I think it is a stretch to say that one possibility is West overtaking the first heart and cashing TWO spades, allowing a ruff-sluff for down only one. East supported spades in the auction and it is quite likely he has four. I told the N-S players that I had considered the options and that I had concluded that the result was not affected by UI from the comment.
3) Does East deserve a penalty or a warning for leading the spade at trick four? Perhaps, but in this scenario the nature of online bridge differs from table bridge. In a club setting, I would have arrived at trick one and I would have been able to warn East about taking advantage of the UI. Online I did not have that chance--when I was called the deal was nearly over. Plus, in an individual, maybe one can make the case that East should not be penalized for West's misbehaviour: if there was a difference based on the spade lead, you might adjust the scores so that West gets the score resulting from a non-spade lead. But I see that Law 89 says this is not allowed.
The key here is communication. You tell the offender what penalty he is to receive and why, without making a show of it. You look at the deal and give serious thought to what might conceivably have happened if East led something else at trick four, as the Laws require--and you tell the players that you have done so, even when there is no adjustment. Too many Directors (most of the ones I mean don't read these forums!) don't want that sort of responsibility. (They should read Law 81).