Halt growth will just stall your city at full food bar. As soon as you remove it you will grow the next turn (even if you do not have surplus food/turn). The unhappy citizen will
- eat 2 food/turn (but hey, you had useless excess food, so this is usually not a big deal. Notice that the city can start starving if you only had 1 excess food, thus killing this unhappy citizen. The annoying thing is that it will subsequently take a long time for the city to regrow, which might mean that you can't use this extra guy if you increase the happy cap in the mean time)
- be available for whipping, both by increasing the maximum number of citizens possible to whip at once, as by being the first guy to get the axe when you do whip.
- Increase city maintenance (it scales lightly with city size)
- Increase trade route yield (both to and from the city; only relevant if the cities population is above 10). This difference is generally less than the difference in maintenance mentioned earlier.
- I believe this unhappy citizen won't eat food when building settlers/workers, but I'm not 100% sure.
In general, unless you are whipping, the unhappy guy is a negative, but only a small negative which takes too much micro to get rid of. The cost of forgetting to remove the "avoid growth" button for just 1 turn after increasing the happy cap is much larger. Also having a few spare unhappy citizens allows you to increase the happy cap by more than 1 and immediately use both new citizens.
The best option is usually swapping tiles thinking "food is worthless, I better work that ocean tile than the bananas", but if you have extra food even when you work the tiles providing most hammers/commerce, than I would not advocate for using "halt growth" unless you really micro-manage everything.