You actually can, just not from the web interface. I do it through e-mail all the time (xymon mailack script or something like that). Assuming there's a xymon command to do so as well, that's surely another way. I believe the issue is that the webpage does not display purple events. Probably an easy tweak.
From: Betsy Schwartz [mailto:user-c61747246f66@xymon.invalid]
Sent: Tuesday, November 26, 2013 01:15 PM
To: xymon at xymon.com <xymon at xymon.com>
Subject: [Xymon] Feature - Ability to Ack purple?
Does anyone else think it would be useful to have the ability to ack purple?
On the one hand, I understand that purples are outside the existing model of how acks work. On the other hand, operationally, without ACK there's no way to indicate that someone has taken ownership of a purple and is working it, other than signing it out. And we hate signing things out because that hides information.