1. a meta-level: all designs of systems have tradeoffs. There is no perfect system. So, by decentralizing we may have to tradeoff the ability to halt trading, in exchange for getting more openness, transparency and neutrality of the marketplace or its operators.

2. specific design: i think there may be ways of achieving this even in a decentralized fashion.

Many use-cases of smart-contracts (i.e. a program run in a blockchain transaction) have envisioned the need for having an "oracle" or external data source. For example, flight-delay insurance contracts (e.g. on etherisc.com) that payout in case of a flight delay would need to get information from some flight-info APIs. To serve this need, there are projects for building decentralized-oracles (e.g. https://chain.link).

Similarly, one can imagine the SEC creates an oracle through a system like ChainLink. This oracle has an api that indicates whether some asset's trading status is ON or OFF. Smart-contracts running the marketplace/exchange on the blockchain have code that reaches out to the SEC oracle before proceeding with a transaction to exchange or sell some asset (i'm describing this synchronously, but it could be done asynchronously).