As far as I understand, your question is related to the urls, generated for the forum threads, created from the frontend.
There are settings, that describe the characters, that should be replaced when generating urls.
Please go to Settings>Advanced>System>Site URL Settings>URLRulesSServer. There you will find the regular expression, defining the characters that should be replaced.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Sitefinity CMS Ideas&Feedback Portal
and vote to affect the priority of the items