This might not be the answer you want to hear, but if the Client paid for it then they own it.
Was there a contract written that specifically restricted the Client from adding the Access Db to other machines?
You might consider adding a license key table to the Db and having it verify a key in the table, then using code make the database valid so that it only goes through this process once.
I did something similar to this for an HR department Db. The departments were required to enter the company name and address when they first used the Db. When they opened the Db, a module would run to check to see if they had entered Company Name and Address values in the Company Table, if they did, the database opened, if not the ENTER COMPANY NAME AND ADDRESS form appeared.
You could do something like that. But remember, you will need to make adjustments like locking out the F11, Shift and "X" keys, and converting it to an MDE or users will get into the back-end.
HTH