Hello Yuriix,

Please try the following code:

/**
 * Check if user can post activity
 *
 * @return bool
 */
function buddydev_user_can_post_activity() {

	if ( bp_is_group_activity() ) {
		return true;
	}

	// Replace ids by your allowed user ids.
	$allowed_user_ids = array( 24, 2 );

	$can = false;
	if ( is_super_admin() || in_array( get_current_user_id(), $allowed_user_ids ) ) {
		$can = true;
	}

	return $can;
}

add_action( 'bp_before_activity_post_form', function() {

	if ( ! buddydev_user_can_post_activity() ) {
		ob_start();
	}

} );

add_action( 'bp_after_activity_post_form', function() {

	if ( ! buddydev_user_can_post_activity() ) {
		ob_get_clean();

		$css = '#buddypress form#whats-new-form {display: none};';

		wp_add_inline_style( 'bp-legacy-css', $css );
	}
} );

add_action( 'bp_enqueue_scripts', function () {
	if ( ! buddydev_user_can_post_activity() ) {
		$css = '#buddypress form#whats-new-form {display: none};';

		wp_add_inline_style( 'bp-parent-css', $css );
	}
}, 15 );

/**
 * Filter ajax post request
 */
function buddydev_restrict_post_update() {

	// Always allow site admin.
	if ( buddydev_user_can_post_activity() || function_exists( 'bp_nouveau' ) ) {
		return;
	}

	$object = empty( $_POST['object'] ) ? '' : sanitize_key( $_POST['object'] );

	if ( ! $object || 'user' == $object || bp_is_user_activity() ) {
		exit( '-1<div id="message" class="error bp-ajax-message"><p>' . __( 'You are not allowed to post activity.' ) . '</p></div>' );
	}
}

add_action( 'wp_ajax_post_update', 'buddydev_restrict_post_update', 9 );

Please replace the old code with this and give it a try.

Note: It will hide the posting form on the activity directory as well. But users can post group activity within the group.

Regards
Ravi