sick to death of no unlimited option im outa here

[#1]

i set my extra bandwidth limit to £15 i get 2 you have exceeded yor maximum blah blah emails within 10 minutes. i was at work nobody home all pc's switched off my wireless is very secure i think its 64 characters long. so i wasnt hacked and now its becoming a regular thing bye bye plusnet

[#2]

Let's look at this logically shall we?
Which package are you on?  That way we can identify what your usage allowance is.
What do you use the internet for?
If you take a look at https://portal.plus.net/view_my_broadband_usage/index.php and  then go to the "usage breakdown", does that tie in with what you believe you've been mainly doing?
I note you state your wireless is "very secure" as you think you've got a 64 character password on it.  Have you actually logged into the router and checked what is connected?
If it's becoming a "regular thing" as you state, then either you're on the wrong package for your usage requirements or you have a security breach?

[#3]

If your wireless HAS been hacked, moving to another provider isn't going to help you. So let's start by making sure you are secure.
You could first have a look at the devices connected to your router (from the router's admin screens) to see if anyone who has hacked your router has given themselves away. You may also find evidence in any connection logs that your router maintains. But I doubt anyone who has compromised your router would give themselves away so easily.
The length of your password has little to do with how secure you are. You need to be running WPA2, using CCMP (not TKIP) and with WPS disabled [not all routers support WPA2 with CCMP, not all will allow WPS to be disabled]. All that WITH a decent password, and if you think you have been hacked, you need to change it even if it is 64 characters long. Also change the admin password on the router, especially if that is still the default. That should see you secure.
Then do also have a look at your usage vs. your package as suggested by geofftswin.