Unfortunately, self-employment tax is charged on all income from self-employment (income reported on Form 1099-MISC in box 7 is considered an item of self-employment income and net income from a trade or business) regardless of whether the person who earned the income is also a recipient of social security. 

Self-employment tax is 15.3% of the income from self-employment. This is the amount on line 12 of Form 1040.