Change the password for your Apple ID. You may also want to change the primary e-mail address you're using for the ID.
If you pur personally purchased apps on a phone owned by your employer, did you also set up an iCloud account on it? Did you activate the find my iPhone service? If so, you could attempt to initiate a remote wipe or remote lock.
Keeping in mind that legally, you may be in big trouble if you wipe the data on a phone that does not belong to you.
Other than that, if the phone was not passcode locked, then there's nothing you can do to keep them from accessing the data/apps that are on the phone now.
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