This question cannot be answered in the abstract. First, talk to your designer about what rights he/she may have in the font.
Next, a lawyer would want to see the contract between you and the designer.
Then you need to contact a lawyer to review the agreement between you and the designer. If the designer had not rights you may wish to contact the font owner and obtain your own license.
Then you need to protect the IP you have purchased, either with copyright, trademark or a combination. This all needs a coordinated strategy based on the specific facts.
Answered on May 28th, 2015 at 8:16 AM