Can you remove fire alarm devices and a fire alarm panel if the customer has not paid for it?

It depends upon the contract, but generally you cannot remove anything permanently installed in anyone else's property unless you have a court order.

In a typical payment dispute, a contractor would place a lien on the property, as a means of securing future payment, then negotiate a payment plan with the debtor, or take them to court for an order of payment and possible seizure of other property.

Others have said:

Yes, you can remove devices but it will add to your cost and would not be cost beneficial. However, you have to use other means.