Fidelity calls its cash a 'core position'. For a traditional IRA account, whenever you add any cash, it will automatically be moved to a SPAXX balance. I got the panic too when I was trying to do a backdoor Roth IRA and saw that traditional IRA cash was moved to SPAXX automatically. I did reach out to Fidelity customer care and they confirmed that we cannot stop or change the SPAXX in traditional IRA. The system automatically moves all cash to SPAXX.