Just have a soft salary cap and a hard salary cap. Can't go over the hard cap or below the soft cap.
By the latter, you are referring to a salary "floor", not a soft cap.
The NBA actually has all three:
salary floor - teams are required to pay out at least 90% of the salary cap. This year, that amount is 89M. If they don't pay out that much in salaries, the difference must be made up in extra payments distributed to the players.
soft cap - this is the 'salary cap' that is most frequently talked about. Non-Bird external free agents can only be signed with room under this cap. It is possible to go over the salary cap by various types of transactions (most often by using Bird Rights and other exceptions) and so it is 'soft'.
hard cap - this is essentially the effect of the luxury tax 'apron'. Once over that, teams lose ability to use various exceptions to sign players. Their salary can still go up, via built-in raises and via Bird Right transactions, but player additions are limited to minimum salary and rookie transactions. This year's apron number is 125M.
Their are more complexities than are worth detailing here, but those are the essential details. If you are interested, please see Larry Coon's excellent FAQ:
http://www.cbafaq.com/salarycap.htm