Sample Code:

<apex:page standardController="Lead" docType="html-5.0">
    <apex:form>
        <apex:pageBlock>
            <apex:pageBlockSection>
                <apex:inputField id="firstname" value="{!Lead.FirstName}" onkeypress="return (event.charCode > 64 && event.charCode < 91) || (event.charCode > 96 && event.charCode < 123)" />
                <apex:inputField id="lastname" value="{!Lead.LastName}" onkeypress="return (event.charCode > 64 && event.charCode < 91) || (event.charCode > 96 && event.charCode < 123)" />
            </apex:pageBlockSection>
        </apex:pageBlock>
    </apex:form>
</apex:page>