Or a vote of no confidence. Or repealing the Act. Or a new law (Statutory Instrument, anyone?) to amend it and effectively neuter it.
Also worth bearing in mind the Tories created the Act too, but now find it inconvenient.
The Fixed-term Parliaments Act 2011 was a condition demanded by the Liberal Democrats in forming the coalition government with the Tories in 2010. Their concern was that as soon as the polls shifted a little in the Tories' favour, the Prime Minister would call a snap election to jettison their unwelcome bedfellows. Before the Act was passed, the Prime Minister alone had the authority to ask the Queen to dissolve the Government and thus trigger a general election, so the Lib Dems would not have been able to stop it.