You check a program setting if user already logged in you immediately log out user. You can use ask question to say user already logged in and then execute log out from the question.
I used this technique in my time clock it checked if user was clocked in if not it logged user out. The effect is it did not let the user log in.
Running actions from ask question buttons is something Ive not looked at yet 
Well I mean you set it to execute a command and set the action from that command
Have only used OK=OK so far LOL
Time to expand your horizons lol.
Know it will be on a post but you couldnāt give a quick example of a button firing an execution command could you?
Ill be able to work it out from a quick example of the button setting.
Guessing something like OK={Automation Command:Logout} - know that wont be correct.
The command name is put into the action there is a spot for that. When you use buttons they hold the command values you want yo pass. Example OK=OK means it will pass OK as a command value of whatever command you set.
So in the rule that handles the command you can use constraints for those values.
I will get thereā¦ only had a couple of weeks really looking deeper at it 
Wow, simpler than I thought, on the list of things to experiment with.
If I digress to the setup I was working on which was ticket only opened on one terminal at a time, is there a way that when the entity is selected to open the ticket (if already open) and I have ask question popup to set ok to be not open the ticket and stay on the entity screen or a ābackā type command?
It would be elaborate setup and I have not tested it. But you could go another route all together and create a new Tables Entity Screen define it as custom and instead of using Entity buttons you could use Automation command buttons and automate the adding of the Entity using the button. What this does is it lets you constrain it before it happens.
I mean the check can be done before the Entity is added to ticket.
To get your States to show correctly you would need to use some ternary scripting in the buttons caption.
That certainly is elaborate in comparison 
But sounds like a good plan.
Might leave attempting that for a bit until more versed is using variables in constraints and actions lol.