Summary: To remove all trend lines, you may delete the GlobalTrendline.dat file from the DASTrader Pro folder
I recently ran into an issue with DAS Trader Pro where anytime I went to insert a Global Trend Line an error message popped up stating that I had "exceeded" the number of Global Trend Lines allowed. After attempting to remove the lines myself without success, I contacted DAS Support for further advice. The result, all Global Trend Lines can be deleted by opening up your computer's DASTrader Pro folder (downloaded onto the computer from the initial download) and deleting the GlobalTrendline.dat file. Then, simply relaunch DAS and you'll be able to place new trend lines!
Hope this helps someone else!