There are 2 different options, depending on the type of protection you need:

 

 

•Protect Excel Formulas or a VB-script - then see Excel protection with DoneEx

•If you developed an Excel Plug-In as a DLL, the protection is possible with Licence Protector, which is a source code integration product - see website www.Licence-Protector.com for details and download the trial version.