In Salesforce Visualforce Page, we can dynamically make a field read-only using disabled attribute.

Sample Code:

Visualforce page:

<apex:page controller="sample">
    <apex:form >    
    <apex:pageBlock>
        <apex:pageBlockSection columns="2">
            <apex:pageblockSectionItem>
                <apex:outputLabel value="State"/>
            </apex:pageblockSectionItem>       
            <apex:pageblockSectionItem>               
                <apex:selectList size="1" value="{!state}">
                    <apex:selectOptions value="{!states}"/>
                    <apex:actionSupport event="onchange" reRender="a"/>
                </apex:selectList>               
            </apex:pageblockSectionItem>
            <apex:pageblockSectionItem>
                <apex:outputLabel value="City"/>
            </apex:pageblockSectionItem>           
            <apex:pageblockSectionItem>
                <apex:selectList size="1" value="{!city}" id="a" disabled="{!bool}">
                    <apex:selectOptions value="{!cities}"/>
                </apex:selectList>
            </apex:pageblockSectionItem>           
        </apex:pageBlockSection>       
    </apex:pageBlock>
    </apex:form>
</apex:page>

Apex Class:

public class sample {
    
    public String state { 
        get; set; 
    }
    public String city { 
        get; set; 
    }
    public Boolean bool { 
        get; set; 
    }
    
    public sample() {
        
        bool = true;
        
    }
    
    public List < SelectOption > getStates() {
        
        List < SelectOption > options = new List < SelectOption >();
        options.add( 
            new SelectOption( 
                'None','--- None ---' 
            ) 
        );       
        options.add( 
            new SelectOption( 
                'TN','Tamil Nadu' 
            ) 
        );
        options.add( 
            new SelectOption( 
                'KL','Kerala' 
            ) 
        );
        return options;
        
    }
    
    public List<SelectOption> getCities() {
        
        List < SelectOption > options = new List < SelectOption >();
        
        if ( state == 'TN' ) {   
            
            options.add(
                new SelectOption(
                    'CHE', 'Chennai'
                )
            );
            options.add(
                new SelectOption(
                    'CBE', 'Coimbatore'
                )
            );
            bool = false;
            
        } else if ( state == 'KL' ) {      
            
            options.add(
                new SelectOption( 
                    'COA', 'Coachin' 
                ) 
            );
            options.add(
                new SelectOption( 
                    'MVL', 'Mavelikara' 
                ) 
            );
            bool = false;
            
        }
        else {
            
            options.add( 
                new SelectOption( 
                    'None', '--- None ---' 
                ) 
            );
            bool = true;
            
        }     
        return options;
        
    }   
    
}

Output: