I don't think you can easily prevent this from happening... You can try a few work arounds:
- shorten some of the tab names, if possible
- Group Tabs into some logical "bundles" and show/hide those bundles based on user choice - for example, a number of tabs under the group "Reports", another number of tabs under the group "KPIs etc...
- hide the tabrow altogether and replace it with your own navigation tools, using ver. 9.00 Actions. With Actions, it's quite easy to develop custom buttons that could serve as a prettier alternative to the standard Tabs.