This is another example of ...

This is another example of the 5 P's:  Proper Planning Prevents Poor Performance.  Intimate knowledge ahead of time with regards to process chemistry has to shed light on which post-treatments are appropriate, rather than some catch-all system.  Now that your system is broken effectively, you come running to others screaming help.  Too bad. 

Go back to the beginning and think:  Why are cationic polymers being used at the point in the process upstream of Ultra-filtration?  Why was UF chosen to begin with?  Is it worth the money to completely re-design the system and start over?  One has doubts, so, you need to examine what gets rid of any carry over of un-reacted cationic polymer upstream of the UF.  Anionic polymers as suggested by others may help, but you are also likely to end up with too much organic "goop" in the water with result being "tar babies".

What do your membrane OEM folks have to say?  Are you going to listen to them?  Are you able to comply?  

Have you considered simply adding an intermediate system such as a clarifier plus clearwell?

If you could at least do the lion's share of polymer excess removal in a clarifier, you might have better results down the road.