I have repro the issue on CRM 2011 + UR13 + IE 8 & IE 10 today but I haven't got any error while closing email template or doing any activity within Email Template so it seems to be issue with either browser settings or CRM. However I created a new environment to test this issue so no customizations / scripts / plug-in exist in CRM.
You have mentioned about IE 8 and IE 10, Does issue reproduce on IE 9? If no, you can opt for IE 9 as workaround for the issue.
If you don't want users to view Email Templates Options under Settings-- > Business-- >Templates then you will need to edit Sales Person security role and under Core Records, remove Read Privilege for Email Templates and save it but remember all users who are having Sales Person security role will not be able to view Email Templates after making changes.
Regards,
Satish T