Hi there:
With my Fidelity SIPP, they simply froze it when their systems flagged up that I was a US person (which I had disclosed to them upon the original transfer from Hargreaves Lansdown). By freezing it, I was unable to conduct any transactions and interest & dividends could not be reinvested.
WIth the Iweb SIPP, they've merely sent a generic email with details of changes in t&c's. I'm going to hold on for now and see what happens - I'm still able to trade at the moment.
I am old enough to take a distribution but can't imagine that they would do that as that would crystallise tax charges - not somewhere they or I would want to go. I'll post an update if and when they take action - their changes to t&c's take place on 24th February. I imagine that if they do anything it will be to freeze the account in much the same way as Fidelity did - forcing me to sell (currently 11 funds), transfer and reinvest.