html部分(增加:disabled="item.disabled"):

<el-select v-model="ruleForm.isXxx" placeholder="请选择xxx">
    <el-option v-for="item in isXxxList" :key="item.value" :label="item.label" :value="item.value" :disabled="item.disabled"/>
</el-select>

data部分(增加disabled:true):

data(){
  return {
    isXxxList: [
      { value: 'no', label: '请选择xxx',disabled:true },
      { value: 'f', label: 'xxx1' },
      { value: 't', label: 'xxx2' },
   ],
 }
}