For what it's worth, that's exactly what the rule is for journalists at The New York Times. To quote the relevant section from the handbook:

"No staff member may own stock or have any other financial interest in a company, enterprise or industry that figures or is likely to figure in coverage that he or she provides, edits, packages or supervises regularly. A book editor, for example, may not invest in a publishing house, a health writer in a pharmaceutical company or a Pentagon reporter in a mutual fund specializing in defense stocks. For this purpose an industry is defined broadly; for example, a reporter responsible for any segment of media coverage may not own any media stock. “Stock” should be read to include futures, options, rights, and speculative debt, as well as “sector” mutual funds (those focused on one industry)."

That would be "all companies."

I was thinking certain employees could deal with certain sectors or something.

Doesn't that effectively translate into "not allowing staff to hold stock at all?"