As your code currently reads.. you may destroy the spite15 every time the layout starts and when his health is 0 or less but as long as the score value is over 20 he will respawn.
Assuming you initialized BossIsDead to 0 when you made it.. otherwise add an action to On start of layout that sets value of BossIsDead to 0;
You need to either add the condition BossIsDead=0 to the Create object Sprite15 on score over 20 event.
OR you could add BossIsDead DOESNOT equal 1 condition to Sprite15 spawn event by creating a condition that says it DOES equal but then right click the condition and choose the red X to invert it to mean NOT.
You will also need to move the System Add 1 to BossIsDead action to ABOVE the destroy action so he doesn't pop in for a split second before the variable stops him spawning.
You add more conditions to events by right clicking the small area behind the current condition and choosing Add another condition... you can make many conditions inside a single event.
Also not sure what the last System BossOneDead=0 is doing.. nothing at this point.... clear as mud?