With the assumption that the person interfering with the play was not connected with either team. There is no rule in the book addressing game interruptions of this nature...not by team members or spectator connected to a team.
The obvious possibilities are neither supported by rule nor supported by the the spirit and intent of the rule.
- Doing nothing (the shooting team was put at a disadvantage not intended by the rules...can't assume that it would have missed)
- Counting the basket (the defending team would be put at a disadvantage not intended by the rules...can't assume that it would have gone)
- Calling a T (on who??).
So, what to do? Whatever the solution, it should be one that gives each team a fair chance to win going forward. With time having expired or nearly so, you couldn't have a alternating possession...that would be equivalent to one of the above...game over.
You could put time back on the clock (enough to run 1 play...say 20 or 30 seconds). Going with restoring time, it's not double jeapardy since we have no idea if the shot was good or not. Give A a chance to run a play, let B defend it.
You could declare an overtime. That could have implications...foul trouble or stamina issues that neither team would have otherwise faced.
I think I'd put time back on the clock and replay the possession. It seems like the most fair and equitable thing to do.