Hi Ekiz,
Please put this code in your bp-custom.php

/**
 * Don't allow changing full name on BuddyPress Edit profile.
 *
 * Pre-process $_POST and overwrite the full name field data to avoid update by users.
 */
function buddydev_overwrite_name_update() {
	if ( empty( $_POST['field_ids'] ) || is_super_admin() ) { //allow super admins.
		return;
	}

	$field_id = bp_xprofile_fullname_field_id();
	$field    = 'field_' . $field_id;

	if ( empty( $_POST[ $field ] ) ) {
		return; //not set.
	}

	// Check the field group exists.
	if ( ! bp_is_action_variable( 'group' ) || ! xprofile_get_field_group( bp_action_variable( 1 ) ) ) {
		return;
	}

	// overwrite with raw value.
	// avoiding displayed_user_name and get_field_data as they provide extra filtering.
	$_POST[ $field ] = maybe_unserialize( BP_XProfile_ProfileData::get_value_byid( $field_id, bp_displayed_user_id() ) );
}

add_action( 'bp_screens', 'buddydev_overwrite_name_update', 2 );

That will do it for you.

You can hide the field using css. User edit’s won’t have any effect.

Regards
Brajesh

Thank you.