Hi Neethu,
Thank you for your interest in Syncfusion producst.
I am not able to reproduce the issue that you mentioned here. I have created a simple sample with spline chart and add a reference line in blue color. Here the unwanted line does not appear. Please refer the sample in the below link:
http://websamples.syncfusion.com/samples/Chart.Windows/F73693/main.htmPlease have a look at the above sample and if still the issue exists, could you please try reproducing it in the above sample and send us the modified sample so that we could sort out the cause of the issue and provide you a solution?
Regards,
G.Yavana