executive will press for highest penalties against offenders
On the windows of the buses of Isle of Man Transport there used to be clear stickers with red letters warning "THE EXECUTIVE WILL PRESS FOR HIGHEST PENALTIES AGAINST OFFENDERS". With the aid of a 10p piece, it was easy to amend this to "THE EXECUTIVE WILL PRESS HIS PENIS AGAINST OFFENDERS". Quite often, it was noted that the executive (whoever he was) had something of a penchant for pressing his penis against fenders, too. Practically every bus on the island carried these modified warnings at some point during the 80s. They don't any more. Pity.
written by Ad*ewl*y , approved by Log

On a similar line, the signs reading "Please mind your head" on our local trains, reminding travellers to watch out for the overhead luggage racks, were easily and often changed to "Fleas in your head".
written by an*nym*us*us*r, approved by Phil

At a pub I frequented as a sixth former, a colleague and I set to work on the blackboard displaying the sweets of the day.

"Black Currant Cheese Cake" became the ever so wrong "Black Cunt Cheese Cak".
written by re* andy, approved by Log

A green felt tip pen can be gainfully employed to change the logo on a Starbucks Coffee cup to read 'Fuck off'.

That'll show 'em.

written by excluded pupil, approved by Matt

In Secondary school we had little 'planners' to write our homework assignments in, and on the front they had the name of the school, Penglais School.
Obviously with Tippex you could easily erase the 'G', the 'L' and the 'A', to make 'Penis School'. One gayer even amended his to say 'Penis Cool', which predictably earned him a severe beating.
written by Dr*b G*een, approved by Mansh

A list of rules on a nearby public Pool Area (including one about 'trespass') was unwisely constructed with those little stick-on letters that are pleasantly easy to peel off. This allowed the creation of the line, "NO ASS IN THE POO AREA". Sadly, a new sign was eventually put up, minus the stick-on letters.

Also nearby was a sign in front of an ice-cream shop advertising 'Buttercream Milkshakes'. The 'er' was stolen from that sign about four times within the space of one day before the store just got rid of it. I had to admire their persistence.
written by an*ny*ous *ser, approved by Mansh

Greengrocers used to use easily wiped off white stuff on their windows? TOMATOES could be partially redacted to TITS (the price of 50p a pound seemed reasonable, and was left alone).

If you're in a hurry, however, GRAPES - £2 can be converted in a single swipe. Although it's kind of defeating one of the key benefits if you end up paying two quid.
written by Ia* W, approved by Log