Allow users to use the trial level only once.

pmpro-customizations.php

<?php
/*
Plugin Name: PMPro Customizations
Plugin URI: https://www.paidmembershipspro.com/wp/pmpro-customizations/
Description: Customizations for my Paid Memberships Pro Setup
Version: .1
Author: Paid Memberships Pro
Author URI: https://www.paidmembershipspro.com
*/

/*
Only allow users to use the trial level once.
Add this code to your active theme’s functions.php
or a custom plugin.
Be sure to change the $trial_level_id variable in multiple places.
*/
//record when users gain the trial level
function my_pmpro_after_change_membership_level($level_id, $user_id) {
	//set this to the id of your trial level
	$trial_level_id = array(
		3,
		4
	);

	if( in_array( $level_id, $trial_level_id ) ) {
		//add user meta to record the fact that this user has had this level before
		update_user_meta( $user_id, "pmpro_trial_level_used", "1" );
	}
}
add_action( "pmpro_after_change_membership_level", "my_pmpro_after_change_membership_level", 10, 2 );

//check at checkout if the user has used the trial level already
function my_pmpro_registration_checks($value) {
	global $current_user;

	//set this to the id of your trial level
	$trial_level_id = array(
		3,
		4
	);

	if ( $current_user->ID && in_array( intval( $_REQUEST['level'] ), $trial_level_id ) ) {
		//check if the current user has already used the trial level
		$already = get_user_meta( $current_user->ID, "pmpro_trial_level_used", true );

		//yup, don’t let them checkout
		if ( $already ) {
			global $pmpro_msg, $pmpro_msgt;
			$pmpro_msg = "You have already used up your trial membership. Please select a full membership to checkout.";
			$pmpro_msgt = "pmpro_error";

			$value = false;
		}
	}

	return $value;
}
add_filter( "pmpro_registration_checks", "my_pmpro_registration_checks" );

//swap the expiration text if the user has used the trial
function my_pmpro_level_expiration_text($text, $level) {
	global $current_user;

	//set this to the id of your trial level
	$trial_level_id = array(
		3,
		4
	);

	if ( $current_user->ID && in_array( $level->id, $trial_level_id ) ) {
		$text = "You have already used up your trial membership. Please select a full membership to checkout.";
	}

	return $text;
}
add_filter( "pmpro_level_expiration_text", "my_pmpro_level_expiration_text", 10, 2 );