Keri,
Happy belated Thanksgiving!
It's been about 3.5 months, so just wanted to give a quick update on this in case anyone else is still having this issue.
Unfortunately, we haven't really seen a solution to this but have improved our situation.
During the period of September 10-17, we contacted Perfect Audience to see if they were experiencing the same security issues as AdRoll. They confirmed that they too had seen some customers affected by this invalid traffic and so their engineers worked on modifying their tag to "no longer track IE7 traffic" and "blacklist all domains" that the traffic was coming from."
Unfortunately, this had no positive effect on our client's traffic issues.
Half-way through October, we saw invalid traffic spike once again into the thousands and stay there until around mid-November.
Half-way through November, the client migrated to a secure site (HTTPs), which actually made a notable difference in the invalid traffic, causing it to reduce down to, what we might consider, normalized numbers.
Although this seemed to help, unfortunately it still didn't completely address the problem. Today, we're still seeing IE7 as the top browser with 95% bounce rate.
At this point, we're just left to run filters to exclude the IE7 traffic until this essentially resolves itself (crossing fingers).