There are many characteristics of disgruntled employees such as the fact that they would be unproductive, they would feel they have been wronged by the company and that the company owes them something, even when that is not the case.
Checkout some of the difficult behaviors they could exhibit and know ways to deal with them.
Some would be thieves
This set of disgruntled workers feel that the company owes them something, so they steal cash, office supplies and intellectual property.
The funny aspect about this is that it is difficult to identify these set of people without catching them doing it on security camera or catching them in the act.
To curb this, you could set a policy of immediate suspension/termination policy for theft or the means for them to pay it back.
The ones that threaten
You need to monitor this set closely especially if after scolding them, you think ill feelings linger.
Signs you need to watch out for are the glaring at co-workers, being loud and aggressive and also sexual harassment.
If after trying to resolve issues with them to no avail, and you think the way to end all that is to fire them, then you need to make sure that the individuals are refused from entering the buildings whenever they want to, and you can also pay them severance packages over a period of months to hopefully calm them down.