Use following class for your VF page

VF page:

<apex:page controller="DateSample">
 <apex:form>
    <apex:selectList value="{!month}" multiselect="false" size="1">
        <apex:selectOption itemValue="1" itemLabel="January" itemDisabled="{!Jan}"/>
        <apex:selectOption itemValue="2" itemLabel="February" itemDisabled="{!feb}"/>
        <apex:selectOption itemValue="3" itemLabel="March" itemDisabled="{!mar}"/>
        <apex:selectOption itemValue="4" itemLabel="April" itemDisabled="{!apr}"/>
        <apex:selectOption itemValue="5" itemLabel="May" itemDisabled="{!may}"/>
        <apex:selectOption itemValue="6" itemLabel="June" itemDisabled="{!jun}"/>
        <apex:selectOption itemValue="7" itemLabel="July" itemDisabled="{!jul}"/>
        <apex:selectOption itemValue="8" itemLabel="August" itemDisabled="{!aug}"/>
        <apex:selectOption itemValue="9" itemLabel="September" itemDisabled="{!sep}"/>
        <apex:selectOption itemValue="10" itemLabel="October" itemDisabled="{!oct}"/>
        <apex:selectOption itemValue="11" itemLabel="November" itemDisabled="{!nov}"/>
        <apex:selectOption itemValue="12" itemLabel="December" itemDisabled="{!dec}"/>
    </apex:selectList>
  </apex:form>
</apex:page>

Apex class:

public with sharing class DateSample {
    public String dec { get; set; }
    public String nov { get; set; }
    public String oct { get; set; }
    public String sep { get; set; }
    public String aug { get; set; }
    public String jul { get; set; }
    public String jun { get; set; }
    public String may { get; set; }
    public String apr { get; set; }
    public String mar { get; set; }
    public String feb { get; set; }
    public String Jan { get; set; }
    
    public String month { get; set; }
    String[] monthArray;
    
    public DateSample () {
      Integer monthInt = System.Now().month();//8
     
      monthArray = new  String[]{ 'jan',
                                  'feb',
                                  'mar',
                                  'apr',
                                  'may',
                                  'jun',
                                  'jul',
                                  'aug',
                                  'sep',
                                  'oct',
                                  'nov',
                                  'dec' };
                              
     for (integer i = 0; i<monthInt; i++) {
       if (monthArray[i]=='jan') {
         jan = 'true';
       }
       if (monthArray[i]=='feb') {
         feb = 'true';
       }
       if (monthArray[i]=='mar') {
         mar = 'true';
       }
       if (monthArray[i]=='apr') {
         apr = 'true';
       }
       if (monthArray[i]=='may') {
         may = 'true';
       }
       if (monthArray[i]=='jun') {
         jun = 'true';
       }
       if (monthArray[i]=='jul') {
         jul = 'true';
       }
       if (monthArray[i]=='aug') {
         jul = 'true';
       }
       if (monthArray[i]=='sep') {
         sep = 'true';
       }
       if (monthArray[i]=='oct') {
         oct = 'true';
       }
        if (monthArray[i]=='nov') {
         nov = 'true';
       }
       if (monthArray[i]=='dec') {
         dec = 'true';
       }
     }
    }
}

 write method for set selected month(enabled month) using follow code stuff

 Integer monthInt = System.Now().month();

 -Suresh