look at the source code and have an attribute disabled .

you set el-submenu :

<el-submenu index="1" disabled>
    <template slot="title">
      <i class="el-icon-location"></i>
      <span></span>
    </template>
    <el-menu-item-group>
      <template slot="title"></template>
      <el-menu-item index="1-1">1</el-menu-item>
      <el-menu-item index="1-2">2</el-menu-item>
    </el-menu-item-group>
    <el-menu-item-group title="2">
      <el-menu-item index="1-3">3</el-menu-item>
    </el-menu-item-group>
    <el-submenu index="1-4">
      <template slot="title">4</template>
      <el-menu-item index="1-4-1">1</el-menu-item>
    </el-submenu>
</el-submenu>