It seems it is an ios bug.
Please go to this plugins’s block-unblock.js and remove line 5,6 & 7 (remove following code)
if ( ! $( element ).hasClass( 'fep_user_blocked' ) && ! confirm( fep_block_unblock_script.confirm.replace( '%s', $( element ).data( 'user_name' ) ) ) ) {
return false;
}
Then clear your browser cache and test.