Poll taxes and unequal protection
A poll tax was often used to prevent minorities from voting - they were disproportionately poor, so all a state had to do was to implement a tax, thereby making participating in an election too costly for the vast majority of the affected group. Making voting expensive was used as a tool to prevent a class of people from doing something they otherwise had a constitutional right to do.
The 24th amendment made this unconstitutional in federal elections and the SCOTUS extended it to state elections in Harper v. Virginia Board of Elections (1966); specifically they (SCOTUS) referred to the Equal Protection Clause of the 14A as justification.
It's no revelation that taxation is used as a means of social engineering. Let's say that a state wishes to abolish something - say cigarettes. Rather than banning cigarette manufacture/sales/etc, they put a $100 tax on every cigarette sold. This has the net effect of banning cigarettes (except for the truly wealthy).
Is this constitutional? Is it a type of poll tax? One could argue that no one has a constitutional right to cigarettes, as one does with voting, but the effect is the same - a state (or even I guess the fed) targeting a group of people to purposely inhibit their rights (here I guess "right" could be interpreted as freedom).