No you can not delete them, I read on here somewhere that these don't count as "items" on you total count. I have several accounts so what I've been doing is send all suits and helmets to second account. You can send 50 at a time. On the museum cards you can send them to second account and "delete them" by not accepting them. But you still don't gain anything give 5 away get 10.
Cars don't count as items either, the maximum number you can have is 2000 though.So they dont count towards the 7,000 items cap, will have to go through my items list and check maybe have to delete some cars from garage, I have been sending outfits over to sub account for weeks thanks for reply