Use FrontPage validation where you can restrict the characters
entered.
Right-click on the form field, choose Form-field properties, click the
validate button.
Change the Data type dropdown to Text
Then check the text format boxes as required. For letters and numbers
only check the Letters and Digits boxes, leaving the others clear.
Check Whitespace to allow spaces and carriage returns. Check Other
and type in other characters, such as hyphen etc.
Click OK when finished.
Note that FrontPage will name the form, and any custom
validation/JavaScript you have fort the form may no longer work.