Prevent users from checking out for a level they already have, unless the level is expiring soon and the user can renew.

pmpro-prevent-checkout-for-current-levels.php

<?php
/**
 * Prevent users from checking out for a level they already have,
 * unless the level is expiring soon and the user can renew.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

// Warn the user if they already have a membership level in the checkout levels.
function my_pmpro_checkout_set_message_has_level() {
	global $current_user, $pmpro_level, $pmpro_checkout_level_ids, $pmpro_msg, $pmpro_msgt;

	// bail if not active user.
	if ( ! is_user_logged_in() || ! pmpro_hasMembershipLevel() || ! empty( $pmpro_msg ) ) {
		return;
	}

	$checkout_levels = array();

	// get the checkout levels
	if ( ! empty( $pmpro_checkout_level_ids ) ) {
		$checkout_levels = $pmpro_checkout_level_ids;
	} elseif ( ! empty( $pmpro_level ) ) {
		$checkout_levels = array( $pmpro_level->id );
	}

	// get all membership level ids for user
	$user_levels    = pmpro_getMembershipLevelsForUser( $current_user->ID );
	$user_level_ids = array();
	foreach ( $user_levels as $user_level ) {
		$user_level_ids[] = $user_level->id;
	}

	// get the checkout level ids matching the user's membership level ids
	$has_levels = array_intersect( $user_level_ids, $checkout_levels );

	// bail if user does not have a membership level in the checkout levels
	if ( empty( $has_levels ) ) {
		return;
	}

	// Let's let the user know they already have the level.
	if ( empty( $pmpro_msg ) ) {
		foreach ( $has_levels as $has_level ) {
			// get the array key for the user_levels where have level matches the object->id
			$key   = array_search( $has_level, array_column( $user_levels, 'id' ) );
			$level = $user_levels[ $key ];

			// Skip if if the user is renewing an expiring level.
			if ( pmpro_isLevelExpiringSoon( $level ) || pmpro_isLevelFree( $level ) ) {
				continue;
			}

			// Set the message.
			/* translators: %s: membership level name */
			$alert_message = sprintf( __( 'You are already a member of the <strong>%s</strong> membership level.', 'paid-memberships-pro' ), $level->name );
			pmpro_setMessage( $alert_message, 'pmpro_error' );
			break;
		}
	}
}
add_action( 'pmpro_checkout_after_parameters_set', 'my_pmpro_checkout_set_message_has_level' );

// Prevent users from registering for the same level they already have, unless the level is expiring soon and the user can renew.
function my_pmpro_same_level_registration_checks( $okay ) {
	global $pmpro_level, $wpdb, $current_user;

	// bail if things are not okay
	if ( ! $okay || ! is_user_logged_in() ) {
		return $okay;
	}

	$level_id = $pmpro_level->id;

	// check if user has a membership level and if they do that it isn't expiring soon.
	if ( pmpro_hasMembershipLevel( $level_id ) && ! pmpro_isLevelExpiringSoon( $level_id ) ) {
		pmpro_setMessage( 'You already have this membership level.', 'pmpro_error' );
		$okay = false;
	}

	return $okay;
}
add_action( 'pmpro_registration_checks', 'my_pmpro_same_level_registration_checks' );

// Filter the levels array to remove levels the user already has, unless the level is expiring soon and the user can renew.
function my_pmpro_levels_array_filter( $levels ) {
	global $current_user;

	// bail if user does not have a membership level already.
	if ( ! pmpro_hasMembershipLevel() ) {
		return $levels;
	}

	// get all membership level ids for user
	$user_levels    = pmpro_getMembershipLevelsForUser( $current_user->ID );
	$user_level_ids = array();
	foreach ( $user_levels as $user_level ) {
		// add the level id to the array if the level is an expiring level and is not expiring soon.
		if ( ! pmpro_isLevelExpiringSoon( $user_level ) ) {
			$user_level_ids[] = $user_level->id;
		}
	}

	// Bail if the user does not have active levels.
	if ( empty( $user_level_ids ) ) {
		return $levels;
	}

	// remove the levels the user already has from the levels array if it isn't expiring soon.
	foreach ( $levels as $key => $level ) {
		if ( in_array( $level->id, $user_level_ids, true ) ) {
			unset( $levels[ $key ] );
		}
	}

	return $levels;
}
add_filter( 'pmpro_levels_array', 'my_pmpro_levels_array_filter' );

To block only specific level IDs see - https://gist.github.com/ipokkel/8a44ce3ce7f31e84c1f789878a21a703