I'm taking a look at this and to be honest I'm a little mystified at what you're trying to achieve here. Your next ship event is always going to end up with the variable holding the value Alpha 4 until Alpha 4 is destroyed because it will set the variable to Alpha 1, then Alpha 2, Alpha 3 and finally Alpha 4 before moving on to any other event.
However to answer the question I can understand, it is actually quite easy to disable an every-time(-argument) SEXP if you use a control variable like this
Do Something
every-time
- =
--VariableName[1]
--1
-do-stuff
Stop Doing it
-when
--Whatever
-modify-variable
--VariableName[1]
--0
The variable is now a switch that can turn the every-time SEXP event on and off. It's often common practice to include a modify-variable SEXP in the event so that it switches itself off after you've used it, like this.
Do Something
every-time
- =
--VariableName[1]
--1
-do-stuff
-modify-variable
--VariableName[1]
--0