1:9 What do you plot against the LORD?
He will make a complete end;
trouble will not rise up a second time.
This verse is speaking directly and personally to Nineveh/the Assyrians living there. "Plot" speaks of computing or devising. "Against" is referring to motion or direction, not necessarily physical, of hostile character. So, putting those two thoughts together, the idea Nahum was getting across is revealed. "What do you devise in hostility towards YHWH?"
It is humorous to note Nahum's response to his rhetorical question. It is as if he's saying, "it does not matter what hostile plan you have because you are going to be completely destroyed!" Echoing verse 8, we learn from verse 9 that YHWH will make a complete end of the Assyrians.
The last bit of this verse is quite interesting in the Hebrew. The Hebrew word used here translated "trouble" is tsarah. It can mean distress or trouble, or it can mean a rival female or female enemy. The verse could be read like, "she [the rival] will not rise up a second time." This could be foreshadowing what happened to the Assyrian Queen in 2:7. Or, Nahum may be referring to Nineveh as a female enemy of YHWH saying she will be completely destroyed and not rise up as a nation a second time.
The most logical way to look at it, though, would be to take "trouble" as referring to the distress and havoc YHWH is about to bring on Nineveh. It will only happen once because the nation will not exist ever again. YHWH will not punish them once, give them time to repent, and then punish them again when they disobey. Nahum is saying, "Look, guys. This is it!"