If you get stuck being unable to login to WordPress because of a misbehaving plugin, you have a couple options.
You can login to your WordPress database using command line MySQL client or phpMyAdmin, and from there look for the wp_options table. If you are using a security plugin that changes table prefixes you’ll have to replace wp_ with your prefix. Once there, look for a value in the option_name column called active_plugins. You can find it like this:
SELECT * FROM wp_options WHERE option_name like '%plugin%';
This will give you a list of plugin options which you can examine. If you are using WPMU or WP3 Multisite and this is a globally enabled plugin, you’ll need to then use table name with the ID of the site in the prefix, like wp_2_options instead of wp_options. It used to be that option_name “active_plugins” would contain all plugins, but it appears from examining this that option_name “transient_plugin_slugs” contains plugins. If you choose to modify it, copy and paste the original value somewhere for easy editing and replacement once fixed.
The option I chose for this Multisite plugin that prevented login was to simply rename the folder of it in the plugins directory. That would cause WordPress not to load the bad files, because they were no longer in the expected location.
Once into the admin dashboard page, you can then disable the plugin completely and take whatever actions you need to repair, replace or trash it for good.