There is no delete option on the license window.
You need to run SQL query shown below against the Configuration database after that he Intrust services should be restarted. Change the license name in the query before executing:
DELETE FROM ADCLicensedObject
WHERE LicenseId IN
(SELECT Guid
FROM ADCInstalledLicense
WHERE ADCInstalledLicense.[Name] = 'InTrust Plug-in for File Access')
DELETE FROM ADCLicenseCounter
WHERE [$Owner$] IN
(SELECT Guid
FROM ADCInstalledLicense
WHERE ADCInstalledLicense.[Name] = 'InTrust Plug-in for File Access')
delete from ADCInstalledLicense
where [name] = 'InTrust Plug-in for File Access'
