Deploy a PGV policy preventing the use of spaces (error displayed when trying to ok/close properties window). Note that this would affect all other types of address, as well as SMTP ones.
If spaces are allowed in your environment for other types of address, create and deploy a custom script policy that would check the 'proxyAddress' attribute for strings that start with 'SMTP:', and if so, do not allow spaces, otherwise allow spaces, etc.
This Product Defect (TF00631158) has been fixed in the latest available maintenance release, you can get more information from its ReleaseNotes:
Active Roles 7.0.3 - Release Notes