Q: Can we threaten disciplinary action if an employee comes in sick?
A: In these challenging times, discipline isn't necessarily the most suitable medicine to get people to self-monitor their health, to respect others, and not come to work if exhibiting symptoms of COVID-19 or any other communicable health ailment.
Hopefully all your employees have been reminded of expected conduct when not feeling well: i.e., stay home, consult with your health provider, and let your manager know your status.
Also, employees should be reminded of benefits they are eligible for, including PTO available to sustain them financially if out ill, and therefore they won't feel pressure to come to work when not well, just to get paid.
So rather than disciplining someone, it might be more effective to send them home after a conversation (or maybe have the conversation by phone after they are home) about these points, and the necessity of seeking prompt advice from their own doctor when not feeling well, and the importance of open communications with their supervisor or manager.