My boyfriend keeps making feeble jokes about a wedding dress. I want him to stop. May I say, już wystarczy tej sukienki? I have a feeling that this is not correct. Can someone please give me advice about the correct use of wystarczy?
You mean that you have heard this feeble joke so many times that it's no longer funny and your boyfriend is getting on your nerves by constantly repeating it, right? If that's the case, and I'm pretty sure it is, the most idiomatic way of telling him to stop would be "Skończ już z tą sukienką" (note that it may sound quite blunt). At least that's what I say and hear people say when they're fed up with worn-out jokes.
If, for some reason, you want to use the word "wystarczy", then I suggest that you say "Wystarczy już tych żartów o sukience". It's a more polite version.