Lots of times this'd be useful:

You alert over multiple hacks, to find you double-disabled them. you sort it out between you, but the alert's still up.

You alert a planet, and ask for pop. help. you get it, and want to change it to "guard please"

To name a few.

Simply, let us cancel alerts, and change the text in them. THIS WILL NOT AFFECT THE ALERT TIMER AT ALL.

CLEARER EXPLANATION (quoted):
What I'm suggesting is a way to cancel or edit an alert. If you accidentally alert someone or something, or the situation surrounding them changes, you'd have the option to cancel the alert (removing it from everyone's logs), or editing it, allowing you to change the text that's sent with it, the most obvious example for me is "sab this planet" to "guard while their potential alert expires".

This would a) improve communication and b) help keep logs clean.

Also maybe a setting to delete expired alerts on NPCs, planets, and players would be nice.

(cancelling an alert doesn't affect the timer).

I think you might be proposing a good method of bookkeeping for players, but I'm not entirely sure what you're saying. Can you explain again?

+1, happened to me a few times.
for the planet scenario, I just type "guard now please" in the legion comm.

I'll re-explain it.

What I'm suggesting is a way to cancel or edit an alert. If you accidentally alert someone or something, or the situation surrounding them changes, you'd have the option to cancel the alert (removing it from everyone's logs), or editing it, allowing you to change the text that's sent with it, the most obvious example for me is "sab this planet" to "guard while their potential alert expires".

This would a) improve communication and b) help keep logs clean.

Also maybe a setting to delete expired alerts on NPCs, planets, and players would be nice.

(cancelling an alert doesn't affect the timer).

+1
Was about to suggest the same thing. Too many times noobs alert over badge hits, would be good to be able to cancel an alert. Much easier than trying to get people to not respond to it in chat.

Even better make it an option on the proposed Legion Alert Tab from another thread.

+1 I like this alot and as a sidenote, it would be good if your own alerts appeared on your ships log, as perhaps that would be a way for you to access the alert to change what it says.

This would be a good add to the game, fully on the ability to edit the alert once its been posted.

+1