There is currently no way for you, an individual user, to deactivate your own account. This is because your access and use of Share911 is provided by your employer or a public safety agency.
If you have discontinued your employment and your employer has not removed your account, the simplest solution to avoid receiving text messages is to log in to your Share911 account and remove your mobile number from your profile.
If you have left your school or company, contact the person who manages Share911 at your former employer to remove you from Share911. Otherwise, contact the person who manages Share911 at your current school or company. If they are not available to you, please create a support ticket.