If this were a question purely about english, I would think it should be
The user was forced to click a button
But I don't think that's a good description of what the program is doing.
I think it would be more helpful to say what the button is for rather than discuss the button itself.
On first reading I thought there was only one button, which I took to mean that the purpose of the button was to wait for the user to be ready to continue, which could be expressed with something like
Now that user is ready, proceed to ...
With more than one button, I assume the program is forcing the user to choose from some set of options (with one button per option), which could be expressed with something like
The user chose an option, let's update the message accordingly
As a matter of taste, I would prefer something like
Update message to reflect selected option
I think it's okay for the fact that the user was forced to click the button to be implied by the comments describing the code that displays the buttons, for example
Prompt the user to choose an option
If I read that comment, I would assume that the thread would not proceed until an option was chosen. Unless the comments indicate otherwise, I would also assume that one of the options is to abort (or cancel), so the user is not trapped.
If the user is trapped, a comment should explain why. Although I don't think it's necessary, it wouldn't hurt to explicitly mention that it can be aborted(/cancelled).
JOptionPane is modal, the user isn't just forced to chose an option to continue this thread, the user is prevented from interacting with the rest of the program until an option is chosen.
In my opinion this is bad design, and if I were commenting this code my comments would reflect that opinion. How appropriate that is may depend on who is expected to read these comments. In any case I think it's important to accurately describe the effect, for example
Prompt the user to choose an option (and don't let them do anything else until they do)
Block the user from any other action until they choose an option
(Of course, the user always has other options, such as killing the program or unplugging the computer, but it would be overkill to mention those.)