Dear Brownie
Sorry to hear your enjoyment was tainted by these windbags.
There are several ways you can handle it in the future.
1) Start small (ish) - you need your voice to carry to them over their flapping gums, with a firm and clear 'SHHHH'.
2) If that fails, kick the back of the chair firmly with a decisive "BE QUIET".
3) Your final warning should be "SHUT UP YOU HOARY OLD WINDBAGS".
If all that fails, see the ushers and demand they act (they are actually paid to) or if they are too wimpy, demand better seats.
Another way is to make condescending, obnoxious comments about their personage.
Or, go armed with a bag of jaffas and flick them into the back of their heads. (Target practice at home to ensure good aim).
Or, at half time go and get a big cup of coke or other cold, sticky, smelly liquid and 'trip' on the way back to your seat.
There are some ideas for you to work with.
Anyway, you could write to the theatre and complain about the ushers lack of policing the theatre. You might score replacement tickets.
**Agony Aunt in no way condones the murder of theatre patrons. But a little humiliation never hurt anyone.