Users do not need to be forced to create plain text emails to create/update tickets. Instead, the account associated with processing Service Desk emails (the email account configured in Mailbox settings) may be configured in mail server to accept mail (IMAP or POP3) as plain text.
That way even when the user that attempt to update/create a ticket, sends an email using HTML format, the Service Desk email account configured in mailbox, will receive an email in plain text, and it will be able to process it correctly.
This, however needs to be configured in mail server. Basically, you may force the mail server to process the incoming mail, and be presented to the recipient account you configured in the mailbox, as plain text.
In Microsoft exchange for instance, this is named as Content Conversion. Please consult your Mail Administrator how to configure this in your Mail Server.