You don't give any clues as to where you are. Figs are herbs, technically and tend to respond as such. If you are mainland US then between July 15th and Aug 15th is a good time to prune fruit trees in general if you aren't looking to encourage vigorous new growth and things heal best during this window generally. On the other hand you can cut sucker off at any time that they appear. I would cut if off at ground level, rather than disturb the roots during the heat of the summer. I am not sure what you are describing. If you expect as single trunk rather than something more herb like then you will have to work at keeping the plant from doing what is natural to it. That is new "suckers" will keep appearing from time to time.