Sorry to hear about your SmartTag being stolen! If it’s a Galaxy SmartTag or SmartTag+, unfortunately there’s no way to remotely disable it if someone else has it. Once it’s out of your Samsung account range, the person can reset and re-pair it.
As for ringing it — you usually need to be within Bluetooth range (around 120m/400ft in open space, but less indoors). If you're close enough, you can use SmartThings Find to ping it or try the AR tracking (for SmartTag+) if it's nearby.
I'd also suggest marking it as Lost in SmartThings Find — this alerts other Samsung devices nearby to help track it anonymously.
Hope it turns up!