Halting a fermentation that is in full press is unlikely at the best of times. Yeast REALLY like to ferment. About the only way I can think of to predictably halt a fermentation would be to heat treat the mead until all the yeast were dead... and that would affect the taste of the mead for sure.
So from a conceptual standpoint we don't really halt fermentation. Instead we let the yeast ferment all of the available sugars ("run dry") until the SG is down around 1.000. Then we add our KMBS and sorbate. Once all the sugars are gone the yeast retool to use other organic molecules for carbon metabolism. The addition of sulfite makes it difficult for them to return to sugar fermentation, as an energy path, once sugar is present again (through a molecular mechanism I still haven't been able to track down though).
The order that most folks will do this is: primary fermentation until dry, cold crash for 7-14 days to promote clearing, rack onto KMBS and sorbate, age/adjust flavor in the secondary. Allowing 48h ish is likely reasonable for KMBS and sorbate to do their thing; though, I imagine most folks are like me and let it sit longer than that (benign neglect is nice to mead once out of the primary).
If you are anything like I was, at your stage, you are currently fretting about your mead oxidizing. Thankfully we need not worry that much as mead resists oxidation fairly well. Its in the secondary, where your mead will spend months, where oxidation can become an issue. Thankfully minimizing headspace and KMBS help protect your mead during that time.
Keep meading my friend!
