Houston I have a problem! My site is running away!

My site is adding about 100 new users a day, about half are creating subdomain sites:

I cannot access my wp admin panel at all and it appears it is because I have exceeded these limits.

Hostgator says: you are exhausting php's memory limit. I recommend setting up caching for this multisite and possibly removing and unused plugins and themes.

How or what can I remove using ftp to regain access to my wp admin area? Can I remove some of these site?

If I do gain control again, what should I change to limit the high php usage?

Stuck!

Cheri Faline