Hello Lyss618
Thank you for your post.
No, a pro-rated refund is not possible. You can cancel the half fare travel card by the end of its subscription year, at least 30 days in advance. In order to do this, you can login on www.swisspass.ch and do the cancellation online, or you can call/email the SBB Contact Center (0848 44 66 88/halbtax@sbb.ch). Another possibility of course is to visit an SBB ticket counter at any railway station in Switzerland.
I wish you a nice afternoon,
EleonoraS