Well, you can lower them some, but this is the thing that plagues open-pole pickup users who slap. Sometimes you have to modify your technique to keep the string from hitting the pickup poles.
I've owned a $$ Warwick Corvette, and slapping always had this result. I'd rather change basses than change my technique. You could also roll to the rear pickup only when slapping.. that may help some, but you may not get your desired slap sound when you take away the front pickup.
Another option is to install closed pickups. That way, if the string hits the pickup, you won't hear it nearly as much, if at all..
I love Warwicks, but I've found that the $$ are the least optimal for slapping. For fingerstyle or pick playing, they are amazing.