Am I able to delete some submissions to get back under the limit?

Hello, 

I am currently using the free service and received an email letting me know my account has been disabled due to me going over the limit. I would like to know if I'm able to delete some of those old submissions so I can remain under the limit? 

Your account disabled due to exceeded upload limit. As a free account, you have 100MB upload limit. If you delete submissions until the uploads reduced to below 100MB your account will be enabled again. You can follow your limits from the usage page.
I recommend you download all submission before delete. You may need them in the future. 
Guide: How to Export All of Your Data at Once
Guide: How to Delete Form Submission Data