Hi Caroline,
yes, you can't delete the default choice, but in fact you don't need to. There are two ways of dealing with the default step: You can use is as the second choice or delete the content from it.
1. Choice A: Selected A in my form - send email A
Default - send email B
That would be a correct setup if you have only the values A and B.
2. Choice A: Select A in my form - send email A
Choice B: Select B in my form - send email B
Default: Do nothing
"Do Nothing" is set if you delete the content from that input field.
Hope that helps,
Michael